As for how to train a "go sniff", it's pretty easy, even if your dog isn't getting it in the real world. Fake it to make it.
Get some smelly food - a piece of roast meat for example - and make a show of putting it on the floor. Although, don't actually put it there, just rub it gently on the floor to get some scent there. Then give him the cue (and make it obvious what you mean), so he goes to sniff. Obviously, there's nothing there, but he'll enjoy the sniffing. If he's not that bothered, do it in long grass and hide a small piece of food in there, so he has to sniff it out. Hey presto, you end up with a sniff on cue.
Side note: in this situation, using the word "cue" rather than "command" makes even more sense than normal. "Command" means "I demand you do this". Well, it's a sniff. Do you care if he doesn't? Hmmm. Whereas the word "cue" derives from the Latin "quando", meaning "when" - should be familiar to you, Emily

"Cue", or "quando" is "when can I do the thing I want to do?". So giving him a "cue" to sniff is entirely more sensible to giving him a "command".
Anyway, I digress.
Once you have a solid "go sniff" cue, you can work on reinforcing the end of it. Let him sniff that pre-made scent, then when he lifts his head, click and give him the food. Repeat. You're reinforcing the end of the sniff as being great. And then, again, once you can predict when he's going to lift his head (because you have a really controlled and predictable environment), you can give your cue to stop sniffing before he raises his head, at which point you click and reinforce. Build up to more distracting environments. In the meantime, use management in the real world, where you can't control it. If that means putting a headcollar on him (properly introduced) then so be it. It's part of a management strategy rather than a long-term solution. Not my cup of tea, but you have to look at your situation and decide what is going to set you both up for success. The head collar can be the cue for "no sniffing now". Take it off to let him have a sniffy walk.