Well from my experience 17 months is too young. There are some kids who have TT that young though. You need to really watch him and see when he's doing something in his nappy and watch to see if he is aware of what he is doing. I've reseached this a bit and it's got to do with nerve endings and whether or not they feel it in the bladder before they wee. Also a big part of it is the willingness of the child to do what the parent wants.

You can show him how to sit on the potty so he is familiar with it. He may or may not do anything in it. Lots of praise if he does do something in there but do not force him to sit if he doesn't want to. The last thing you want to do is make him feel unsecure or anxious when he sits.

I've had one child who refused to TT and completley resisted till he was 3 1/2 but then when he got it there were no accidents! My other son was 2 when he started going on the toilet by himself with no prompting from me at all but it was really hit and miss with him. Some days we had totally dry and others he would wet his pants.
He had been dry for about 6 months when his brother was born but started wetting and pooing his pants again - several times a day. It took 4 months for him to be reliable with the toilet again.

2 in nappies isn't that big of a deal. I've done it and looking back it was much easier to manage than TT regression.