Rather than make him joint, you could just add him as an authorized user. This would kickstart his credit report and give him a nice long history. Since eventually FICO 08 will not count authorized users, I'd recommend you do this, wait until it reports, then have him apply for a few cards on his own to build up individual history.
Yes the AU account will report as 1984, before his birth, which is fine. I've got a card reporting 1978 from the exact same scenario.
As long as the new card account has a good payment record, that will not hurt him at all. I'm not sure what you mean about rebucketing. With 1 AU card his average credit age would be 24 years. The only harm would be new cards (under his own name) would factor a 0 into there each time you apply, but of course this is necessary in the long run (after AUs don't count), so better sooner than later.
Doing this should give him a nice FICO, and therefore eliminate the need for him to start off with the subprimes/rebuilders.