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We go full Ellyn Satter. My job is to choose what, when, and where. Your job is to choose whether or how much. You don't have to eat but you do have to pleasantly keep us company at the table. And if you are hungry before bed you may have your dinner plate. It sounds strict, but it actually ends in really pleasant, cheerful meal times, I promise.

If pur 3.5 year old doesn't want to try something we don't push it, but we also don't offer other options that aren't part of the main meal. We only substitute veggies. Like, he doesn't like salad so we don't fight with him on salad nights and just give him peas. (We made him try salad probably 10 times before giving up). Some meals we make more "kid-friendly", like adding some butter to his greenbeans, which I cooked longer just because he likes them that way.
Our other strategies are 1)giving smaller portions of the most liked foods & not allowing seconds until he's taken a bite of everything on his plate. 2)no thirds unless he's eaten everything on his plate. We say "if you're hungry enough for more pork, you're hungry enough to finish your rice." 3)allowing him to refuse any foods without us making a fuss. If he doesn't want to eat, that's fine with us & he knows there won't be any other foods after dinner (we pretty much head straight to bath after dinner).

We are in the same boat, I swear my 4yo gets pickier by the day. He LOVES fruit, and will eat it in massive quantities. It's gotten to the point where I have to not give fruit as part of his meal or snack because he will eat only that, claim to be full and then ask to be excused, only to be hungry 10 mins later.
My usual tactic is to make him chew and swallow one bite of everything and he has to stay at the table while everyone else is eating. If he's hungry again his food from the previous meal will be available.
That used to work swimmingly: I don't know if he truly doesn't like things any more or if this is just a way of asserting control, but he is down to only eating
10-15 things reliably. And he used to be the best eater....sigh

@MrsSRS: We do the same thing. The only thing we add is that if he's asking for a snack or something we're not having, he has to eat so many bites of his actual dinner before he can whatever it is he's asking for. So if he wants applesauce, he has to eat 5 bites of chicken or finish the veggies on his plate.