I've done that several times. The cost is five half-tones at the top end, and who ever played those notes anyway?!?! Not me!
Most 4 strings have a 34" scale, which in my experience doesn't usually yield the most solid B string tone, but in many cases it works just fine. You won't know until you try it on his bass, and for the price of a set of strings it is worth the gamble. Buy a good set of strings for a regular 5 string, and use the G string for furniture repair, or sell it on ebay!
The nut slots might need to be slightly widened, but not always. The BEAD strings will probably have a little less tension than the EADG strings the bass normally sees, so you may need to relieve a little of the tension on the truss rod to prevent the neck from bowing backward. You'll probably need to touch up the intonation as well, but this will give you and your son an opportunity to do something together, and bond.