Training - When can I enroll him?

This is really a personal decision about weighing the risks and benefits of early classes. There is less of a risk of disease in a controlled environment like puppy class, but there is still a risk (when not all pups are fully vaccinated). The school near me took pups at 8 wks and so we were there by the time mine was 9 wks.
 
That said, from my experience I would say the primary purpose of going at that very young age is socialization more than training. Puppies that young have the attention span of fleas and though they certainly can begin to learn, it's not like at 4 or 5 or 6 months any bad behavior's going to be set in stone (far from it). You have plenty of time for training, less for socialization. However, if you prefer to socialize your pup in other ways that can work too, as long as you make the effort to do it.
 
I don't regret our early puppy K in the least, but most of its benefits were in areas other than strict obedience training - it was useful to me to hear about other puppy owners' experiences, I learned a lot about training and body language, I learned to be patient and tolerant with my puppy, I bonded with my puppy, and he got to play with other puppies. In terms of *learning commands* I think we got more out of our subsequent homework and follow up "graduate puppy" class.
 
Hope that helps!

The "socialization window" starts to close at 16 weeks. If your puppy is already 15 weeks old, and hasn't had A LOT of interactions with other puppies and dogs, you need to get moving. You can train a dog at any age-- but if you don't socialize them properly when they are young they cannot learn those skills later.

My puppy is 5.5 months.  We got her from the breeder at 16 weeks.  She was with littermates and adults till then.  Then she came home and made friends with our 2 adult dogs, and mothers adult female.  We are going to get her to school soon.  I hope she has been socialized enough already with dogs?  I mean she isnt' screwed up is she?  We've been taking her out to Petsmart and the park lately. 

If she was with her litter until 16 weeks that was good doggie socialization time. Most people have a situation where they get a puppy at 8 weeks, take him home and he's an only dog and is not around other dogs much during that window of time. I would certainly continue to expose her to new situations, places, and people but don't fret about her time w/ the litter, it was well spent.

I got my puppy at 12 weeks, and I think we started puppy class a week or 2 later. I felt comfortable with it because she had already had 2 distemper shots, a bordetella shot, and her rabies shot. She needed one more distemper shot, but I was fine with starting class at that point.
 

Sequoyah started puppy class at 11 weeks, because she was with her litter until 10 weeks (I had to get time off to go get her, because I wouldn't fly her from Georgia).  She did fine, too, even though I would have started her right at 8 weeks if I had her that early.  More dogs die from behavioral issues than from disease, although that's a risk.