I often get lambasted for this, but I'll say it anyway; snuffle mats and puzzle toys can be a nightmare if overused, not used properly (not saying either of this is the case with you) or if used on the wrong dog.
We often get dogs into the rescue I help with, and they are so often described as 'manic' or 'unable to settle' and what they all have in common is their owners use food-games (kong's, puzzle matts, snuffle mats etc). Once we get them into foster and remove these games...it's like having a different dog. Sadly, IME, these games do not work for an awful lot of dogs.
I'd cut out any of those types of games, if you're doing them.
What I cannot recommend enough, is training. Start by giving your food slightly less food with his meals (to avoid over-feeding) and instead use those calories in another form (chicken, beef etc) for training. With mine, when I WHFH, I do a quick 10/15 minute training session at lunch and one again about 16:00 ish. Training done properly is exhausting for dogs. If your dog can already do most basic commands, start trying to train harder ones - extended settles, stopping when running towards you (emergency stop), back-up (moving backwards), extended stay or down, and then doing all of these at a distance. So perhaps you standing in your kitchen, getting your dog to sit, and then going out the room (peep around the corner if you can) and seeing how long he will sit for and then return before he moves.
Part of it is his age though, at 15 months he's got another 12 months nearly before he's an adult. They get bored quickly at this age so you have to tire them out in the right way and get them used to settling without toys or stimulation - or you end up with a wired manic who never learns to settle.
I hope that's helpful! 😊
One of the Spaniel owners will be along in a moment (I hope) and can talk to you about scent work if you're interested in it!