Parents PLEASE help me!! EXTREMELY picky eater

My toddler is 3 and his older brother is 4. Their 15 months apart. My 4 year old is picky but will eat most things (meats, eggs, dairy, veggies, fruits, snacks) it’s just a small amount of things that are a no go for him. Now… my 3 year old (as a mom) I’ve always felt was a little different. He’s very emotional. He mostly eats yogurt, fruits, grits, mashed potatoes(if I literally feed it to him), beans and rice( sometimes) and sweets if available. I cannot for the life of me get him to eat meat, eggs, veggies, or anything with genuine nutritional value. He’ll eat grits with butter, cheese and a tad of milk mixed in all day if you let him. He’s losing weight.. idk what to do. I’ve brought it up at appointments for a year now and they keep saying it’s normal. He’s now pale and light. He was 33ish pounds and dropped to 28 over the past few months. I haven’t weighed him since his last appointment since they said he was fine. I’m honestly so scared to even weigh him. I feel like I’m not doing everything I can.. I’ve tried smoothies, veggie straws, V8 drinks, pediasure.. nothing works.. is there some sort of medication he can take to pick up his diet? Should I take him to the pediatric ER and demand a full panel of bloodwork/everything to be tested?