My 3yo son is way short. Allergic to milk & soy. How do I help him grow?

So my 3 1/2 year old boy recently started preschool, and he is by far the smallest kid in his class. I already knew he was on the short side, but it seems like even more than half of the two year olds are bigger than he is at 3.5. He's allergic to both milk & soy (plus a whole bunch of other random stuff).... show more So my 3 1/2 year old boy recently started preschool, and he is by far the smallest kid in his class. I already knew he was on the short side, but it seems like even more than half of the two year olds are bigger than he is at 3.5.

He's allergic to both milk & soy (plus a whole bunch of other random stuff). I know I should be glad that he's at least got all his fingers and toes, but I keep reading all these posts from teenage girls here about "this short guy is asking me out, how do I turn him down without seeming like a shallow b*tch" and it breaks my heart thinking my by will be going through that in ten/fifteen years.

I'm not particularly short (though not tall either - just about 5"10), and my wife is 5"6, so neither of us have experience being the smallest. And he gets along fine now with his peers. He's got a good personality and I'm not as worried about bullying as I am about being rejected by girls over his height.

So what can I do to help him grow if milk & soy are out of the question? Vitamins? Exercises? Anything I'm not thinking of? Thanks!
Update: ** edit: Allergic to goat milk too. Thanks for the responses so far, all!
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