you can buy foil containers with cardboard lids, like takeaways come in. You can get them in poundland sometimes, Robert Dyas and often in asian and kosher shops. They go in the freezer then into the oven (take lid off before putting in oven). You do need really to defrost then reheat (put in fridge the night before). I did a fish pie from the freezer like this last night but from frozen. It took and age and it got a bit dried out.
Alternatively you can get plastic takeaway containers with lids. they seems to be smaller and more prone to leaking, but have the advantage of going in the microwave. I have seen these in Poundland.
I'm not sure that a roast dinner would freeze and reheat that well.
I made meals for DB and SIL for when they had a baby recently. I did spag bol, chilli and rice, fish pie and casseroles. I did separate containers of mash for the casseroles. All froze and reheated well.
Pieces of roast meat like you'd use in a stir fry, mashed potato and gravy, and frozen veg can re-heat well. The texture of other spuds goes a bit mashy. I'm assuming microwaving from frozen - if the person is up to tipping contents into a frying pan and heating and stirring, then texture survives better.
Shepherds/cottage pie is good, fish pie, chili and rice, curry... The plastic containers from takeaways can be found in pound shops/Wilkos/ethnic supermarkets and only really soupy stuff leaks. My favourite is enchiladas made by my mum, with a couple wrapped in foil ready to go in the oven to heat.
A roast would freeze fine if you used mashed potatoes rather than roast. Make sure there's plenty of gravy on the meat to stop it drying out.
I second the foil tray suggestion.
notcitrus has covered most of what I'd suggest but also a good casserole/stew - you could put potatoes in it so that it's a one-dish meal. Lasagne would also freeze well.
If he's coming out of hospital and his appetite may be all over the place some soups might be a good plan. Things like watercress and potato are filling and full of iron. Minestrone would be a filling lunch. Soup options are endless!