Take all the advice you can get, and I would definately get an instructor to assist you with lunging for the first few times, until you and your cob get the hang of it. We have a young horse, we bought on from 2 years old and she is now just rising 4, we have started lunging her for the last couple of months. We put side reins on, but just very loosly to let her get the feel of reins on the bit rings. again with saddle etc on.
Does you cob know voice commands to stand, walk on and trot on, preceeded by his name. If not you can do this in hand whilst walking him about the yard, or practise this in your menege etc. Have a look at a book on lunging, you can buy some cheap ones, and do not have to go really expensive. Best to buy a basic book, or read some websites and have some lunging lessons with a good instructor. Again let him have a natural head carriage, he will come up when he is feeling more balanced. It will not take forever to be able to use side reins. Only do not do too much lunging i.e. day in and out as running around in circles can be very boring for them. To help out could put a small pole on the circle or small jump to give them something else to consider whilst going around in a circle. Remember that if you normally ride for 40 minutes that 20 minutes of lungeing is the equivilent. Also just start really slowly and for few minutes only. Make sure you always do both reins for equal times, so as he does not become better on one side than the other.
Once you have mastered one rein, think about long reining, you can lunge with two reins too.
Good for you and keep chipping away with you horse, he will soon improve. Better to think what you can acheive in a year with steady determined work, than trying to acheive something in say two months. Give him and yourself time to learn together. My cobs have both been very forgiving with me learning with them. But it has been worth it, as if it had not been for my cob, I would not now have the skills to bring on our younger pony
Good luck, sorry this is so long!!!