I work with people experiencing homelessness and we ask people not to give money directly as it is often used on alcohol and/or drugs - kind people who think they are helping are often unknowingly feeding the addiction that many people experiencing homelessness are working hard to beat.
Of course not all homeless people are battling addiction, but sadly a very large number of them are and direct money donations, as generous as they are, aren't the most beneficial for the majority of people experiencing homelessness.
What a lovely kind child you have raised. I would personally suggest he offers his time instead of his money and helps out at a local shelter/soup kitchen - however, please bare in mind that many of them won't allow under 16s to volunteer at all, even in the back, and if this is the case with your local ones, then perhaps he could spend his money on donations to take in instead. Make sure you check with them which items of food they need most beforehand.
There will be ots of local charities you could Google and contact to see what donations they are most in need of. We often get inundated with gloves, hats, scarves etc at this time of year - which if course, is lovely, however we are often left with boxes full come the end of winter. There are often other items that we really struggle to get donations of, this will differ between charities so contacting them and asking directly what they need is your best bet. For us personally, we always seem to need more; sanitary products, shoes and underwear.
I definitely think your best bet is small local charities. Best of luck and a massive well done to your amazing DS!