He doesn't complain about things upsetting his stomach, but he does have a thing about textures. My husband picked up some Pediasure powder at Boots yesterday and we gave him some after breakfast. He drank some and we put the rest in the fridge for after school. I'm definitely not putting any pressure on him. If he drinks it, great, if he doesn't, no big deal.
The time limit thing is because he is the biggest dawdler you will ever meet. Like, taking 10 minutes to eat a single fish finger. Dinner was taking so long it was delaying bedtime! (It's not just dinner, he's like that with most things. Walking anywhere is an epic journey because he has to look at every car and ask questions about everything! I generally don't fuss about it unless we have to be somewhere because I like to encourage his curiosity.) We're not making him wolf it down in 5 minutes. It's more like, after 30 minutes if he's still just dilly-dallying, I set a timer for 10 more minutes. He can eat whatever he likes in that 10 minutes, but when the bell goes off, dinner is over. If he's had a decent meal, he gets dessert. If not, "we'll try again tomorrow," as he likes to say.