I don't know that he has a specialty per se. He's an advanced research scientist. He's good at all that sciency stuff. Whatever the writers need him to be good at this week.
More seriously, he's probably got an MD-PHD. Dual doctorates in medicine and... probably physics. Or maybe biology, with an undergrad background in physics. Or something like that.
When it gets down to it, though, he's a nerd. He likes science. Understanding how stuff works. Knowing all the cool things you can do. He works in a building full of nerds and swaps information with them all the time. Lunchtime at STAR Labs should probably be worth college credit. So, he's aware of stuff outside of his field because he's interested in it and, whatever it is, the guy down the hall is probably an expert in it.
