The support group idea sounds good. To find an in-person group in your area, the Food Allergy Research & Education (FARE) site has a page to help find one.
If you can't find one in your area, perhaps an online support group would be helpful for you. A forum where you can get ongoing interaction, suggestions and moral support from other parents. I don't know these personally, but they look worthwhile checking out: http://community.kidswithfoodallergies.org/forum/main_forum, http://www.allergychat.org/allergy_forum/
And to find an in-person group in your area, this site purports to help find one: https://www.foodallergy.org/support-groups
Over time, you may find it helpful to track some numbers,weight and there'sheight numbers. There is a nice calculator that gives you percentiles online at http://www.infantchart.com/Baby Infant Growth Chart Calculator; I put in your son's numbers and got the 10th percentile.
I wonder if it wouldIt could be helpful for his doctor to give you a referral to a nutritionist.