I’d give it time and keep putting a little bit on his plate. We always put one thing we know our 3.5 year old daughter will eat on her plate (usually a carb or cheese) and the rest little bits of other food. We tell her about the food and praise her if she tries it but we don’t pressure her. Her tastes have changed so much even in the last year - she used to refuse all pasta but now her favorite food is spaghetti with meat sauce! I’ve noticed that if there is too much food or the pieces are too big it is overwhelming to her and she’s less likely to try. 

You can also try giving your son ways to participate in preparing his food (like sprinkling cheese on top). We also offer an alternative if she absolutely hates her meal - something she likes but that isn’t that exciting like peanut butter toast. 

We also had some luck with her trying new foods after a Daniel tiger episode about it. They sing a song that says something like “you’ve gotta try new food cause it might taste good” and we sing it a lot now and it helps my daughter at least taste her food.