what should I be looking for to realize it's time for my son to have longer cranks? He's rode 155's for over a year, seems to still be comfortable on them, but I feel like maybe it's time for 160's just because he's been on 155's for a while now. THOUGHTS/SUGGESTIONS ? FYI..my son is 9 with a 23" inseam

If he doesnt look cramped up dont change them.

This is from jrbicycles.com. It is just one of many you can find by googling Crank length charts. Most are very close, but they do vary some. And remember, these are only one persons interpretation of the correct crank length. Some people will feel better spinning a shorter crank, while others will feel better lugging a longer crank with a taller gear. It's all personal. Also, if you decide to go to longer cranks, give him a week or two to get used to them before you decide whether or not they are adversly affecting his performance. It can take you some time with the new cranks to get used to them and get the best out of them.

Crank lengths

also, what's the advantage of outboard bearings ?? are they beneficial to the mini/junior sized riders?

Probablely not. Little muscles don't produce a lot of power.
Although I suppose if the frame designer had them in mind, then use the outboard bearings.
keep in mind that the may increase the width between the pedals (q-factor).
You do not want the pedals to far apart as this can cause knee and hip pain.

I also concur on not going with an outboard bearing bottom bracket, mainly for the additional weight added. Not only with the bottom bracket, but also the crank arms required for this type of bottom bracket. I would suggest using a titanium square taper BB, (FSA, Crupi, Sinz) to take advantage of the weight savings on the BB and crank arms. My 9x has run this setup for more than a year with no problems. I remove the BB about every 3 months and do a thorough cleaning.