How great that he sits and waits!
That is the first huge step!
I would build on that and just get him to stay until released.
For the dog's safety, you can use a martingale collar, that if fitted correctly, is nothing more than a flat collar that tightens if he tries to slip out. (It should not be fitted like a choke collar!)
I have trained my dog only using this type of collar. My trainer only uses these type of collars---even with mastifs and all big paws! Once my boy realized he couldn't slip out of it, he became more compliant.
What we have worked on (for a year now) is 'loose leash' control. The collar isn't used for pulling the dog away (this can create dog agression)- but just to be sure he is safe and can't run away.
How we trained a longer stay is to:
In a non-distracting place- put him in a sit and every 5 secs, praise and give him treat for holding it for 1 min or as long as you can the first few times.
Then pat him on his side to 'release' him (I say FREE)...give him a few minutes break and then repeat for 4 times...on 4 consecutive days.
Then you try to get him to sit for 10 sec for 1 min; 4 times, etc. until he sits for 1 min.
Then you do this in more distracting places.
The idea is conditioning..
.repetition--dogs learn in 3-4 times in a row....weird- eh?
Think of training a behavior you *do* want to replace the behavior you don't want.
http://www.sitstay.com/store/equip/collars3.shtml