He is sixteen and has decided not to let us know he needs to go out

Time for a trip to the vet. At that age, he may not know he needs to go out until it`s too late.

SOmetimes when dogs get older they lose control of their bladder. Just think of your dog as an old human being... A lot of older people will lose control of their bladders as well. THere is a medication out there that can help, but, I wouldn't advise giving such an old dog medication that could possibly kill his liver/kidneys or any other important organs.

There are doggie diapers out there to help you with this problem as well... That can get expensive though...

Talk to your vet about all options... I know there are quite a few out there.

I don't think he's decided, I think he just can't help it.

As with older people, older dogs can lose control of their bladders.

It's time to get him checked out by a vet to rule out infections.

As lucky mentioned if it is just an age problem diapers can be a good option.
You can buy special products but for males it's a lot cheaper to make your own. (with females it's a bit harder)
A friend of mine has Shih Tzus and she uses very wide elastic bands and places a women's sanitary pad over the dogs penis, then the band around their waist... no more mess and cheap if you buy just generic pads.