That happened to Nikkia a few months ago I had her trained to do over 30 commands then one day she just stopped and acted like she didn't know anything! I figured I'd give her a break for a little while then we would work on re-leaning her tricks then when we went back to it she could do all of them again, I think sometimes like us they just get a little overwhelmed and need a break.
Some tips to help out with not overwhelming him are keep training sessions short me and Nikkia only work on her new tricks 15-20 minutes then if she is doing well or not we still take a break for a couple of hours before starting up again. Because as you have probably read and heard from many people dogs learn through frustration if you hold that treat over his head he is going to try everything he can to get it and trying to figure out what it is he needs to do can as you can well guess be quite frustrating. So long training sessions let the dog get more and more frustrated and some times they'll think this is not fun it's so hard to get that darn treat and I've been trying for so long I'm just not going to do this any more! And like Victoria and Jedi said always keep things fun and happy because the more of a game it is to him the more fun it will be and the more willing he will be to learn!
Pack leader - Yes eat first I have Nikkia sit outside the kitchen and she watches me eat then after I am done I get her food and take her to her eating area then she has to sit and wait while I put her food down and walk away then when I am on the other side of the room I release her and she can eat. I feed her twice a day once at 6 am and once at 6pm it takes 12 hours for dog food to digest so right after she eat she goes strait outside because eating will stimulate your pooches instincts to go to the bathroom.
The sit thing is a good one to only I don't like to use one command all the time because it seems like that just becomes their trademark thing and they always do it I also like to keep Nikkia on her toes and make sure she is always ready to do what I say so instead of making her sit before we go outside to play maybe one day I'll have her roll over and the next I'll have her be shy. Also when were playing maybe every third or forth time I throw her Frisbee or ball I'll have her do a trick for me first and I won't throw it until she follows all the way through no halfsees.
Also when you walk him make sure he is always next to or behind you in the wild pack leader is always in front because he is the one who knows where to go he is also the first one to react to anything that comes up. If you let your dog walk ahead of you then that gives him control and he gets it in his head that he is the leader. Also when he comes up to something unfamiliar he will react instinctively sometimes resulting in being a little aggressive mostly out of fear, where as if you react first and he sees you as his respected leader he will react the same way you do.
Set down solid rules and be consistent! You should always have and follow through with a set of rules so that he knows how you want him to behave in your home. Such as he has to sit in the other room while you eat dinner make him do that every night don't let up! If you let him get away with things he will try to get away with more and more until he is walking all over you. Think of it this way do you respect someone who doesn't follow through with things? Well neither does your dog.
Socialization - This is probably one of the most important things you can do for your dog teaching him how to interact with the world around him is critical! When I was socializing Nikkia one of my favorite places to go was pet stores like Petsmart and Petco. Because number one they are allowed in and number 2 they are exposed to dogs and people of all breeds and ages. We would just go around with a bag of treat and I would ask random people if they could give Nikkia a treat let her sniff them and pet her. I also had Nikkia do specific things before she could be pet like sit down before she was pet by adults and older children and lay down when she was petted by toddlers. Dog's you have to be careful about always make sure the dog is calm and okay with other dogs before you introduce your puppy to him because having dogs snapping at him will turn him fearful towards them and we are trying to achieve the exact opposite of that.
Hope this helps you out.
I would love to answer any other questions you may have.