There are quite a few things you can try. The first might be to try a diffuser from your vet, which contains pheremones which help to calm dogs (honest- we use it for fireworks!) and helps them feel more secure. It does seem to work really well for a lot of dogs (and for some reason my brain is still enjoying its maternity leave and is refusing to remeber the name, but your vet will know what you mean!)
You can also try a few drops of rescue remedy in his water- this seems to help my old boy with fireworks.
Just a thought, but often older dogs bark when they start to lose their hearing. No idea why, but I think the vibration is somehow comforting, so may be an idea to leave something (?) humming in the background!
If it is purely behavioural, you can try some behavioural therapy. Simple stuff like putting your coat on, then going nowhere. The dog gets anxious then realises there is no need (you feel a berk, admittedly, but it can help!) I also used to go out, wait a short time then rush back in, so he isn't sure that you going out means you have actually gone. You can also find, despite many owners being averse to the idea, an indoor crate or kennel gives your dog a den and a feeling of security. As a last resort there are drugs specifically for this problem (clomicalm, eg.), but in older dogs (and esp. boxers, with their funny cardiovascular quirks), I prefer to avoid drugs unless absolutely necessary. Hth!