Hi Jim. I changed to 5 string in the early 80's. I only really use the low B string to play notes below the 4th. E string. Literally I never play the 5th. string above the 5th. fret. I am not a fan of 6 string basses. There are some fantastic 6 string players, Anthony Jackson being a prime example. I think your son will benefit from the 5 string.
I hd a jazz bass when I first joined F.C. but it was stolen. Dick Knight made me a birds eye maple one and also the body for the strat. which I later sold to Simon. The only Jazz bass I have now is a Squier 4 string "Jaco". Best wishes, Peggy.
Interesting....My favourite 6-string player has to be Phil Lesh. I found this on an interview site somewhere:
"as asks: Phil: when did you switch to the six string, and why? Do you ever play the 4 string anymore?
Phil_Lesh_Live says: There's just more notes on the 6 string and they're easier to get to. It has a extreme low register that I like. It's tuned in fourths, so everything is just right under your fingers. "