Erin,
A couple different responses.
About your son's eczema, has he been tested for allergies yet? My daughter had really bad eczema on her face and we found out much later that she had allergies. She is allergic to eggs, peanuts, milk, wheat, the whole range. I am not one of those moms that says he does; however I didn't know that the shots babies receive are egg-based and she was allergic to eggs, therefore prolonging the eczema.
Second, I don't know if you use Aquaphor on his eczema; however we give my daughter a bath and slather her up (when she is still a little wet) and put her jammies on; face to feet. Her skin is amazing now!
Finally to your real question, I found that Carters worked for size and most are 100% cotton which is good for his skin too. Our pediatrician said to stay away from polyester so the skin irritation could begin to clear up.
I wouldn't try to buy clothes by the months, I would go by the weight or heighth. My daughter was born at 10.2 and stayed in the 95% until she began to crawl/walk (which, by the way, crawling and walking can come later than others because they have to lift and carry a little more than others.)
Good luck!