A., I can somewhat relate to what you are going through. But I have girl who will be 4 yr next week & is tall for her age. She has grown 2 inches in 4 months, will eat almost anyone under the table, is going into size 7 clothes. She is thin, but not as thin as your son (from the sounds of things). She is about 38# & 43" tall as of Sunday. My husband & I are not tall, but our grandmother's were.
I always get, boy she is tall for her age (yes she is, & she was a premie by 6 weeks). She is very active- she wears us out!!! From the people who see her frequently (parents at her school) they usually comment on how much she has grown in a short amount of time.
We also get questions about where she gets her blue eyes (mine are brown green hazel & dad's are green) & her blonde curly hair (dad's is really curly but short, I had blonde curly hair as a child). And consider, that I wanted to adopt (just because) but my husband wanted the adopted child to "look like us"- Caucasian so there wouldn't be any questions. That plan got blown out of the water.
I just give brief explainations- we both carry blue eye, blonde hair genes, & has tall genes in her family, & she eats plenty. Then I kind of blow them off. I don't think most people mean to be mean, I think they just want to strike up a conversation. I often tell them she will be the next super model & Derek Jeter will be my son-in-law in 20 yrs.
Sorry I can't help with any advice on the clothes issue.