B.P.
That is very thin for a boy! My friend's son is 3 and just got weighed in at 39 lbs and that is 75th percentile! I have the same problem with my son. It helps when he sees other children eating something and also when he is preoccupied with something he likes. My son has started to say No much more often lately because I think he likes the feeling. Maybe someone else can offer him the food? I would keep the food plain, like no stews or chunky soups unless that is his thing. Maybe he can help you cook? Also, you could tell him he is tries a bite you can do something he likes. I have found that my son is much more agreeable when he can clean the litter box after wards (he loves this). I also try to make the eating fun my telling him to make a big bite or a little bite. It can be very time intensive. I am no expert and I am looking forward to tips from other moms too!