Hi @Beansandcheesearegood - what does he guard?
You're dead right that he's probably growling at your husband because he has form for snatching. Spaniels (and retrievers too) can very easily become resource guarders as they're bred to hold things in their mouths and that's often all they really want to do. He hasn't got any idea what's his and what's stealing - dogs don't think in that way.
Before I got Bill, my best friend's dog had a resource guarding issue, because the kids would leave things around, and the dog (an Aussie Labradoodle) would pick them up, and the kids would snatch them off her. And of course the more they did that, the more value they placed on all their discarded stuff that the dog then wanted to steal.
That's why it was on my radar, and when Bill was a puppy I taught him a swap, and I almost never take anything forcibly from him. If he steals anything that isn't actively dangerous or genuinely delicate or precious, I let him, and after a while I'll go and swap for a treato. There's a lot of trust between us now, and while there are occasional treasures he won't swap, this is pretty rare, and he's never growled; I can get the car keys, the bread knife, my shoes, and the remote controls off him for a very small fee. If I don't come after him, he tends to bring them back and look at me as if he's asking where his treato is.
That's why I'm asking what he's stealing and guarding - for the most part, your absolute best bet is to just let him. Resource guarding tends to start because the dog is afraid that things that are precious to him will be taken away. If you have precious and dangerous things, put them high up, and otherwise, just let him take stuff. Everyone in the house will have to be on board though, you can't have one person grabbing stuff out of his mouth.
You could then teach a swap for things that aren't so precious (e.g. give him the cardboard recycling and swap for treatos). It's also handy to have things that are valuable to the dog but not to you - Bill used to love a clothes peg above everything else, and now will do almost anything for a plastic bottle.
As others have said, and especially if you've got kids, it's a good idea to get a behaviourist, as resource guarding can get dangerous. But they're almost certainly going to start by telling you not to take stuff off the dog forcibly, to always reward him when he gives things up, and that if you want things left alone, you have to put them well out of the way.