He's just a baby! To be honest, I find putting step-ins on young or rambunctious dogs are more difficult. I DON'T find them easy at all, lol. Jackson luckily just naturally lifts up his paws and knows what to do but when I was dogsitting, step-ins were a pain in my @ss for wild-ish dogs, or even dogs that just... were no help at all and I had to lift their legs up into it, and then by the time I got one leg through a hole, the other has already slipped out LOL...
Anyway, first things first, you need to train the dog to enjoy their harness. Here are some tutorials.
Easiest on/off I've had is the Ezydog harness, but it's a bit high up on the chest for some peoples liking. I really love Puppia as well. And I do love Buddy Belts and ChokeFrees (leather) but def think you should wait until closer to full grown...