My ds is like this, and it drives me mad. The most encouraging thing I can offer is that he will not starve through stubbornness. if you have a good hv (mine is fab) it can be worth asking for reassureance, height/weight, but I'm sure you will hear a similar phrase!
I still have a ds who will only eat mash if it is the least of the evils on his plate and tells me he doesn't like something before it's even dished up when I know he will eat it!
tbh, just make it clear that the meal in front of him is all that is on offer. you say you've been offering alternatives, so it could be a bit of a control thing, esp if he eats the same meal at nursery. Also don't sweat it in the kitchen over his food, ok so he's more likely to eat it if it looks nice, but really, why stress if there is a good possability you'll be nuking later for his supper.
I usually offer fruit in between meals if they get whingy, if they are seriously hungry they will eat it and so get at least 1/5 of their 5 a day
If his skills are enough, you could try offering choices before you cook, ie pasta, pesto and sausages, OR sausages and mash. it offers a little control, but you are still the boss. Don't forget the skills of hiding veg either, my ds has eaten shop pesto with a small amount of mashed broccoli and loved it...ha ha! But I would advise putting a portion of 'normal' veg on the plate so he is used to seeing it, a couple of pieces of carrot etc, sometimes they love the strangest of things, my dd loves roast parsnip! Keep his portions small. less waste, and not so overwhelming. he might be willing to try the odd bite from your plate then the same from his own, but be careful with this, don't let them get to a point where their whole meal is eaten from yours!
Sorry, I've gone on a bit, but my son is a nightmare foodwise, so I thought I'd pass on a few tricks
Good luck