Something I do frequently is garlicky mushrooms. As a side dish to a steak, served over hot (sourdough!) toast, or on a bed of rice or stirred through pasta (penne or fusilli are great). Or can be added to a basic risotto.
Chop and slowly fry (in olive oil and a knob of butter) 2 cloves of garlic. Toss in a good handful of thyme leaves if you have them.
Add a carton of regular mushrooms, chopped to a uniform size (size depends on the smallest size in the carton, how nicely you want to present them, and time you have to cook - I like bigger if a substantial meal, or smaller if a side dish). Season well with salt and pepper, and allow to slowly cook. You may need to add some extra butter during the cooking process, but you want them to cook well, and get some lovely golden colour on to them.
As a side dish, they can now be plated up, but for more substantial elements, I often add a dash of white wine and reduce that down, and then a decent splash of double cream to just heat through. Test seasoning. (For vegans, no butter, no cream but should otherwise be good).
If you have any more interesting mushrooms, either add them with the regular mushrooms if fresh, or part-way through if dehydrated (rehydrate them first though in some warm water - and that water makes great stock to add instead of wine, or for making risotto).
This is handy because it is so versatile, and could be your side dish while the main part for DS2.
We've also tried Jamie Oliver's veggie curry (from his "instead of a Friday night takeaway" slot on a money-saving meals series), which was great. Sorry I don't have a recipe to hand.
DH also learned how to do a veggie chilli when we had a veggie au pair a few years ago - basic chilli sauce (onion, garlic, tinned tomatoes, tinned kidney beans, chilli powder etc) but using peppers, cauliflower, onion cut much larger, tin of chickpeas, etc, which was very tasty and we still eat that as often as a beef chilli now.
Is he veggie or vegan? Because if he will eat cheese, teach him a cheese sauce and a tomato sauce as the base for various meals, and he can do lots then. We do cheese sauce based on melted butter, add in cornflour, then stir in the milk to make the roux, bring to the boil stirring, then add cheese (and a bit of English mustard sets off the cheese nicely), and grated pepper. I am rubbish at making sauces based on flour and cooking it out. But that can be used to make cauliflower cheese (substantial in its own right - I often have this with roast potatoes as dinner with no meat, but is also a grand side for the rest of you), or pasta dishes (I do macaroni cheese with (bacon pieces - I know leave those out), fried onions, fried garlic, fried mushrooms (not as long as the dish above), diced peppers, courgettes (I put those in the pasta water to cook for the last 4-5 minutes), French beans, broad beans, peas - whatever I have to hand but a nice mix of veggies), or just plain mac'n'cheese with no veg.
The other thing I do big batches of is roasted meditteranean veggies. I dice up my veg into equal sized chunks (mostly about 1cm, but sometimes I go for more rustic 3cm for less elegant, or BBQ skewers etc). A mix of red onions, garlic, red peppers, courgette, mushrooms, (fresh) tomatoes, (aubergines are good but the rest of my family don't eat them), etc. Mix together with salt, pepper, herbs (thyme, basil, rosemary etc - whatever you have), and a good slug each of olive oil and balsamic vinegar. Leave them marinade for a while (20 mins is enough, overnight is great). They can be roasted now, frozen now and roasted later, or roasted now and leftovers frozen for later. Takes about 25-30 mins at 180 degrees for 1cm dice. Great as a side dish with roasts or BBQs (and larger pieces can be cooked on BBQs on skewers). They also make a great sauce with a basic tomato sauce with pasta (jar or onion, garlic, tin tomatoes, seasoning types), and we often do that with maybe some bacon bits, cooked sausages, cooked chicken or prawns too - so quorn sausages could work well, or fish (if he eats fish?), or taking his out of the pot before adding (already cooked) meat elements for the rest of you. Or the basis of a bruschetta type dish on nice bread. And as the cooked veg does freeze well, that could be a good one to have as a standby (I do BIG batches at a go (I fill the biggest IKEA freezer bag - I think it's 5l?), cook half and freeze half still raw, but of the cooked half, I get a couple of meal-sized batches of leftovers already cooked - so it's a great one for spend 20-30 minutes chopping but get lots of reward from that effort).
A VERY simple one, that I use when I feel like not making an effort, is cheesy leeky pasta. (The leeks are also something that can be made in bigger batches and frozen). Chop the leeks, and fry/saute gently for about 15 minutes with some butter and seasoning and a little veg stock. You want them soft and melting. (They are a great side dish with pork chops!). For the pasta dish, simply boil your pasta, have the leeks hot when pasta is cooked, grate a nice handful of a decent strong cheddar - drain pasta, throw in cheese and leeks together, stir around and allow 1 minute for cheese to start to melt with the heat of the pasta/leeks, throw on the plate.
(My other "go to" dish when DH is away and I want to spend no energy, is M&S welsh rarebit and a couple of part-baked bread rolls, thrown into the oven - no veg, not really great for you, but I think it's better than some of the ready meals in terms of additives etc, and I eat very little bread anyway). Sorry, that one is probably not quite hitting the "proper food" brief!