IMHO Steve, as he often does, nailed it! This is a 7 yr old dog that has been with you for a month! Down is a sort of submissive position, so rather than press him or insist he obeys, I'd also be much more inclined to lure (Bet you didn't expect that from me Steve!) praise and show him how much you appreciate his compliance, rather than trying to "enforce, or even reinforce" it for now. If age or pain is keeping him from complying readily, you'll notice that. But for the next month or two, work only on stuff he's good at and likes, build some more trust and respect. Down is probably something he knows, but you can't assume that, and that's where luring and praise can win him over. Especially with an older dog, I would never advise "enforcing" anything until they've had ample time to adjust...which can take several months in my experience. He sounds like a great dog though, it'll come, just give him time!
It's not the size of the dog in the fight, it's the size of the fight in the dog. Mark Twain