older dog acting strange

Hi bubblypop,
Have a chat with your vet about this. I'd be thinking, given his age and what you're describing, there's a likelihood that this is Canine Cognitive Dementia (CCD). Initial signs include restlessness and agitation, clinginess and increased anxiety about being left alone, disturbed sleep patterns often typified by night-time wakefulness (with more sleep during the day to compensate), forgetfulness, depression.
The good news with CCD is that if you act quickly enough, and get it in the early stages, it can be halted. It can even be reversed, which is not possible with humans, that we know of.
First interventions are dietary, with a supplement called Aktivait, which increases antioxidants and various other nutrients to help counteract the ageing process. This should all happen alongside a number of behavioural interventions which aim to keep the brain active without over-stressing an ageing body. Things like several short walks instead of one longer one, activity feeding instead of feeding from a bowl, retraining some old exercises and maybe teaching some new ones. All of these also help tire the dog out by day, so he reverts to normal sleep patterns. However, sometimes the vet will prescribe mild sedatives on a temporary basis to help knock the sleep patterns back on track, if the activities during the day don't quite work on their own.
But, you do need to talk to your vet either way. I will say that there has been increasing awareness of CCD in recent years, as CCD was often cast off both by owners and vets until quite recently as the dog just getting old. But now we know that this is a process which, for dogs, we can give considerable help.
Best of luck with him now. I hope he's okay. I've an awful soft spot for the oldies

I think, as a matter of course, I'd just assume that it's CCD, as it will do no harm anyway, assuming there's not another underlying condition, but I'm assuming your vet is on top of the physical stuff.
Immediate things you can do... feed him sardines. If he's dry fed, opt for a food aimed at older dogs. Order the Aktivait online as it may take a while to get to you. It's good stuff. And get going with the behavioural stuff too!