You are correct that his weight is below the 5th percentile and his height is between the 25-10th percentile for his age. Are the males in your family having small stature? If yes, then he may be smaller than normal. At 15 months, he can have 'fatty' foods because he brain is still developing and needs fat for it. It is after the age of 3 that children can be feed low fat foods/dairy. To help him gain weight in a healthier way, suggest in place of milk and at the end of each meal give him milkshakes (milk and ice cream blended together – add a banana if he will accept it) or yogurt smoothies (yogurt, milk, and frozen fruit blended together). Add dry milk to oatmeal, mashed potatoes, gravies, creamed vegetables, and creamed soups (add additional meats to soups). This will give him additional calories and protein for weight gain. Use all regular dairy products and no low fat foods so he gets the calories he needs. Hoped this helped you.
I am in the same boat, My son is almost 14 mo old and weighs about 18 lbs. He also eats more than his 3.5 yr old sister. His ped. told us to feed him, "fries, and cookies, and ice cream..all the stuff we avoid when trying to lose weight" but that just doesn't sound very healthy for one and for two, we don't normally buy that kind of stuff and try to avoid fast food...so do I go to mcdonald's just for my 1 yr old when I have 2 more kids that I don't want eating this stuff??