I'd say give the son the Python because it's the only real heirloom answer here, and take the money that might be spent on buying a nice gun now and take that trip or do something together that is especially memorable for a 6 year old.I'd say to hell with a gun, take the money and go on a father-son trip.
That said, I sincerely hope the Dad recovers from the cancer and all of this gift planning is premature.