[soah]

I would fold preflop. You don't have enough information about him yet to be sure that he's raising you with crap, and you don't know how far he'll go postflop with something marginal. This makes it difficult to bluff and you can end up in some bad spots when you just have a bad pair.

Given that you've made it this far, I think you need to raise, and probably push, although I'd be happier about it if his bet were a bit bigger or the pot were bigger or the stacks were a little shorter. His bet here could mean a lot of things. He might have a pair with no diamond and doesn't want you to check behind. He might have a big diamond and is betting because he considers this to be an improvement for his hand. Of course, it's also possible he has QT and doesn't want you to check behind with a diamond. He might also have a flush here, but I expect most players to check if they hit it. More often than not I think you're ahead here but you don't really want to see another card.