He's not eating it, and he's not absorbing it thru the skin. I don't see how he could get poisoned.

Also, remember, it's not like eating an ounce of lead will poison you.

Yes, kids eating lead paint over many months can be a problem. But there the lead is ingested, and they are kids.

An adult holding solder in his mouth is not a problem.

FWIW, I did this myself for many years from 13-24 without any problems... and I don't recall ever swallowing a single piece of solder.