He's a baby! He's also frightened, and not terribly strong physically. He can't have had his last jab that long ago so he definitely isn't used to the big outside yet.
Carry him for a bit - then when there's somewhere you can have a bit of a game, pop him down and play for a few minutes. Is he used to a lead yet? That will also likely be putting him off.
Whatever you do, don't force him to walk on lead at his age. Distract, carry, play - whatever it takes.
He can't possibly have a bloody clue why you've put something on his neck to drag him along/stop him doing what he wants, and definitely hasn't worked out it has positive associations with it.
Get him used to the lead, hold a treat in front of his nose - something really stinky - and get his attention by using his name, a high singsongy tone of voice, acting the fool - and get him to walk along beside you while he's on the lead. If you get it right, he will never learn to pull on the lead. Keep him on the same side ALL the time (usually on the left) and don't allow him to weave about.
He needs teaching - it won't take long. At all!