What @linguaphile (and everyone else) said.
I’d ask first what communication method he prefers, then go with that. Learning any language is daunting at first, but the only wait to retain vocabulary (and grammar) is to use it. If you make a good faith effort to use the signs you’ve learned, it shows you’re making an effort to communicate. If he’s as nice a guy as you say, he’ll encourage that and help you out as you go along. But communication with anyone is a two-way street. Just saying “hi” or “bye” or whatever won’t advance communication. So don’t be afraid to aim for more than just simple comments/phrases.
As @filmfann and others suggested, taking a class would be very helpful, if you’re so inclined. Ask your co-worker for suggestions on where to go.
Like @linguaphile said, keeping a sense of humor is probably the most important thing (and I think that’s true for just about everything!) :)
One more thing to add: @gailcalled, lipreading is very inexact, and not necessarily the best option (as I said above, best is to ask the deaf co-worker’s communication preferences!). However, your suggestion of a whiteboard is a very good one, and may help @serenade‘s co-work not just with @serenade, but with work in general.