Question from a Parent...

[damned]

To keep the information overload coming -

There is a new version coming out this year that wont be upgraded to for free. You will not have to change but by the end of the year I would think that at least 2/3rds of games will be running on the platform. But that will only matter if playing in a group that has moved to that platform.

As many have said - for 3.5E look at moving to Pathfinder -it will be very familiar but you will have plenty of source material available and Paizo make fantastic adventure paths.
Do consider throwing off the shackles and moving to 5E - it might also be easier for his friends to get into (or not - 15 is a tricky age!).

Every table will be different in terms of style, content, maturity and much more.
I would suggest you monitor his interactions or group during the first session or two and make sure you are comfortable with the mix.
My online group has always been mostly over 40 years olds (now its split with over 50yr olds too!).
I did have a 16yr old join the group and he played till he was 18 and allowed to go out and drink (Australian).
I regularly asked him over the first few sessions if his parents were ok with his participation.

As far as online communities go this is a pretty good one - but experiences will definitely vary.
Perhaps you might join him and together you can create great stories!

[deer_buster]

I think as long as he knows basic internet safety, he'll be fine. He's probably heard/said more at school than he would ever hear/say at home, so I don't think you need to worry there. I would honestly go into Pathfinder or 5e, as you'll likely find a lot more games available in either of those. If he is a player, he doesn't need to buy any content, and doesn't even have to buy a license in a lot of cases as there are a lot of game masters that own the ultimate license, which only requires a free license of the players. That would be the best place to start. I would highly recommend stopping by Fantasy Grounds College online and in discord, and there are a ton of YouTube videos on playing various rulesets in FG. I would recommend holding off being the game master until he has more experience with FG.

[dendarii]

In terms of versions, you might consider D&D 5e, as many of the really big online streaming campaigns that he and his friends might watch are 5e-based.