[jtr]

If he's got the flush draw with 2 overcards he's a modest favourite, about 54:46. It's quite possible in this case that he only has a suited ace or something, so that would make you the favourite by the same margin. Either way, with the money that's already in the pot you should call, unless you're horrendously deep-stacked and worried about the overcard flush draw.

Is either of your tens a heart by the way?

Of course, all this assumes that you are 100% certain it's a flush draw. If you're wrong and 25% of the time he's got a set, for instance, then fold.