It is possible/practical to sail a Hobie 16 with just the jib ? I'd like to depower it to take out my young son.

No,very hard to sail with jib only.Much better option is main only.Use the jib halyard attached to forstay adjuster to give rig tension.

Difficult if not impossible to sail upwind and or tack with jib alone.

I once saw a couple guys doing it though. Wind was mid 20s to 30s. More like reach, reach.

I was forced to do it once (main halyard broke),do able but like Matt said hard to make ground upwind

Thanks guys. I think the best option is to try to find a 14 main for it.

Does the main have reef points?

Instead of finding a H-14 sail, it might be easier to find an old blown out H-16 sail, then convert it to a storm sail. You could pull out the stitching on the bottom tape, take out a few panels, then re-sew the tape on a now smaller sail. If it is still too much sail, then take out another panel. You got me EXCITED, I just might try this if I can find a cheap old sail!