I didn't mean it in a harsh way, just sick of hearing about walking through a door before your dog means you are showing him who's boss. Mine don't rush through either, they wait to see what I want of, them first, if I say off you go, they go, if I say wait, they wait . They know who rules without you having to resort to these totally stupid ideas, by your behaviour and demeanour you have already shown them your status, hence the reason why some people cannot control just the one dog without a lot of palavar and screaming, the dog knows they are incapable and does exactly what it chooses.
Been out with 3 today, walking along after a run along the towpath, all onlead, passing one dog onlead, Lona did look at it,and Zen wanted to play, Mabs wasn't bothered either way. A brisk Walk On Girls, and they immediately carried on. So did the other dog, as well, no barking, no carry on, no uncontrolled dog swinging from one of my girls faces either. Very sadly that experience is rare, most dogs please themselves what they do

Your daughter being a groomer, or working with dogs, is not the same as a qualified vet checking him over. Whilst your daughter can check for obvious injuries, your vet will go in more depth. For example it could be his joints that are causing him pain and whilst he might seem ok to you only a vet can truly determine that.

Also, you mention he's been so good you've never had to raise your voice, but then state he accepts 'light scolds'. What exactly do you mean? And what do they consist of? To be honest if you've ever physically removed him from somewhere or something with some force/irritation on your behalf his behaviour could well be as a result of that. Not saying you have, but just throwing that out there. If you want him to move or come to you call him to you instead. Want him off the sofa? Walk to the kitchen calling him and offer him a treat once there for complying.

A behaviourist is probably a good idea. But be careful who you choose as the wrong one could do more harm than good and all these people who believe in 'dominating' your dog or showing him you're boss are not worth the paper they're written on. Ask your vet for a referral or find someone local to you here - http://apbc.org.uk/help/regions