how can can I make meal times with my 4 year old son less stressful?

My son was diagnosed with autism last year and I'm new to this forum. I would really appreciate some advice on how to make meal times less stressful.

He tends to prefer soft foods and doesn't like things mixed up (so a casserole which has meat and veg in a sauce is a no go). Meat and veg separated on the plate seems work better; but actually he really struggles with eating veg; he doesn't like the textures. So if we insist he eats even a piece now, he screws up his face and looks like he wants to be sick and can go into meltdown...

Fruit is not such a problem luckily (but he will only suck the juice out of satsumas/clementines pieces - doesn't like the skin) and pears need to be peeled. The texture of food really does cause problems for him. 

We've recently tried putting the food into dishes (rather than plating up for him), so he can help himself, in the hope that it would make things less stressful for him; but now he really will only choose the meat or pasta (beige food) with mayonnaise!

So my question is, if he eats meat, fruit, lots of milk/yoghurts, bread/pasta, but pretty much no vegetables should we just let him be? It's tricky because we don't want his younger sister who is 3 to be influenced by his habits. And ideally don't really want to be cooking different meals for them both.

I think he is generally healthy, but worried he might not be getting enough vitamins etc. I guess we could give him some supplements, but not sure what?

Any advice, greatly received. Thanks :)