Every little bit counts..even a tenner! I'm sure family will help and don't feel shy of asking strangers. You'd be surprised. Many will say no, but some will say yes. A cheery smile, and a short explanation of the aim before asking for sponsorship works wonders. Obviously I don't know your circumstances, but think where you might meet people, school gate, work, even outside local shop or post office. Our post office will take sponser forms and put them on the counter for people to sign. We've recently done a petition to save our bus that way. It is a small community p.o, so easy to do.
Local Facebook event groups. Local paper might do a bit on you to help with publicity and sponsors.
Lad can get sponsors from Teachers at school(assuming he's of that sort of age)
Depends how private you want to be about it. But the more publicity, the more you'll raise the profile of the charity and money for it.
Go for it. Best of luck mate.
Maybe even some here might like to sponsor you.