We started my daughter on a sippy cup when she started solids, at 6 months. It only took her two tries before she knew how to use it and she's been holding it on her own since about 1 week after we introduced it. All babies are different and will adjust with time.
I only offered water in the sippy for a long time, just because juice doesn't offer any nutritional value babies can't get from fruits and veggies. I finally decided a couple months ago(she's now 1),I'd introduce juice (very watered-down) since I figured I'd like to offer and alternative to plain water. She seems to like the water better.
I've tried to put formula in the cup, but I get concerned because she doen't drink as much from that as she does a bottle so I'm still trying to figure that one out.
I would say you're good to introduce the sippy cup to your son at anytime now.