Should i worry about my 4 year old's weight?

A lot of times, the doctor is just looking to see if the child CAN gain weight, just to try to rule out a medical issues that may need treatment. It is not that the low weight in and of itself is a medical concern, but it could be something more serious, and the low weight is a symptom of that. So, if your son does gain weight on teh high fat/calorie diet, then likely all is fine, but the doctor probably needs to know that, just to rule out other things. I would follow the doctors' advice. A high fat diet in someone as young as your child is not going to make him fat later in life, and the doctor needs to know if he can gain weight.