@tylwim12 Thanks for reaching out!
While we are not aware of any organizations that offer kid's mountain bikes as a giveaway or at a discount, you may want to connect with a local biking organization like a bmx club or mountain bike team. Additionally, there are some other options for you to consider: first, keep an eye on re-sale websites like Craigslist or Facebook Marketplace for a quality used bike, you can even post that you are on the look out for one and you may get some leads. Secondly, you can check in with your local REI store to see if they have any returned bikes that have been marked down.
Most importantly, keep encouraging your son to get out and ride!
Hopefully this helps, thanks!