Well, there's a lot to consider, but the fact that your DS is allergic will complicate things a bit.
There's no such thing as a hypoallergenic dog - the non-shedding breeds may mean your DS has less of a reaction, though.
I would say non-shedding breeds need to be your priority, then look at what else you can offer a dog iyswim. Bare in mind that all the non-shedding breeds require (very) regular brushing and grooming appointments which won't come cheap, though you can of course learn to do it at home.
Non-shedding breeds include poodles (all sizes), Spanish Water Dogs, Schnauzers (both sizes), Bichon Frisés, Maltese, Yorkies, Coton de Tulear, Westies, Cairn Terriers and Shih Tzus, though there are plenty of others.
Then look at what exercise you can offer on a daily basis, what qualities you could or couldn't cope with etc.