Hi, I am by no means an expert having just started down this road myself with DS, and I am sure that someone with more knowledge than me will be along to advise. What I can tell you is that it is possible to test for allergies at any age. DS was RAST tested at around 6 months, and skin prick tested last week (he is now approx eight months). Having read every single one some of the threads on here I know that others dc's have been tested even younger and had accurate results. I would want to have tests for at least the 'big eight' allergies as quite often dairy and soya can go hand in hand, for example, as well as egg and peanut. This way you are forewarned if there are other foods that you need to avoid (although of course I hope this is not the case for your little one).
You do need to be sure that the test results are explained to you in context, by someone who knows what they are talking about. Having had DS RAST tested, my GP had us avoiding wheat, when DS's score was only 0.1 over the negative score and showed no reaction at all to the skin prick tests. So I have spent the last three months avoiding wheat in my diet when I didn't need to! On the positive side, I have lost four stone since I had DS, but I would have killed for a piece of toast at times! The tests will not give you any idea of how your DS may react to anything that he is allergic to, only how likely it is that he is allergic, iyswim?
We saw Adam Fox last week, who is one of the top allergy specialists in the country. (This is not in the way of boasting, just to emphasise that I regard the info we have been given as very reliable!) His opinion is that the foods you eat while pregnant and/or breastfeeding have no bearing on your child's allergies. So while avoiding your child's allergens in your diet may save them some discomfort, actively consuming them has no bearing on whether they will eventually be outgrown.
TBH, it has only been through reading the threads on here that I had any idea of what to expect from our appt., my GP was quite happy to refer us but very little use when it came to providing any accurate information on how I should be approaching DS's allergies, weaning etc, etc.
I hope I don't come over as patronising if this is stuff that you already know, as I say we are still in the early stages of the allergy journey ourselves, but just wanted to share with you what I have learned on the way so far 
Good luck!