Don't assume it's a game - that is just putting an emotional interpretation on it and always causes problems in training animals.
Just because it LOOKS playful doesn't mean it is. Dogs, especially working and protection breeds, often try to 'cover up' fear of obstacles and don't always show it obviously. In any case, it's irrelevant whether it's playful or not.
I would say the mistake you make is trying to deal with this when he's off lead.
You have to keep in mind that given how tall some dogs are, or how short, or how they're put together, certain obstacles hit them wrong, and make them just about frantic. I mean literally 'hit them wrong'. My dog got rapped in both stifles every time he jumped up on my old bed. You have to be willing to consider that the way these steps are spaced or whatever, that it's just possible that he is getting a real good bang in the stifles every time he went up them and he quit because he's smart, not stupid.
The OTHER thing to consider, is that this dog is developing dysplasia or puppy lameness (he's at the exact right age for puppy lameness), or some other physical issue. ANY time a dog is reluctant about an obstacle that other dogs in the house are going over, or that does not seem like much of a barrier, consider very seriously, that this can be due to pain.
I would also not leave him out and let him 'get hungry enough to just get over it', or drape a blanket over it - the more excited he is the more likely he is to make the blanket slip and be even less willing to go on the steps the next time. Even if his foot slips slightly, he will be less willing next time.
What I would do, assuming he is not in pain, is put him on a leash indoors, take him out another way, and lead him on the leash, up to the steps.
JUST STAND THERE. Just stand there next to the steps. Use the leash and collar to put him right next to the steps and parallel to them. No petting, comforting, pleading, NOTHING. Just have some treats and give him one once in a while. Just use the leash, and hold him next to the steps, do NOT try to get him to go up yet. Let him stand there for about ten good minutes. Just do that for a couple days. Make the steps a nice place where he gets fed.
Then, again, put his leash on in the house, go out by another route, and take him, on his leash to the steps.
Now depending on if he was hitting himself going up these steps, you may have to teach him a different way to get up the steps. It depends on the geometry of the steps. Steep, tall porch steps really can rap a dog very hard. If they're very low and the issue is clearly that he just does not like them...you just take him up straight, but don't discount the possibility that the way he was going up, he was getting hurt.
But say if it's more a dislike for their look or feel, not actually getting hurt.
Then you spend a few days teaching him to put his front paws on the first step, over and over, for a treat.
Then usually, if you have given the first steps enough time and repetitions, you can walk up into the house, hold out a treat, and call him and lead him up the steps with the leash, and give him a reward.
Do NOT try to fix this off lead. And give him the treat after he DOES something, after the first couple days of just standing by the steps, that is.
Desensitize, reward, then train, reward. Then you're done and the problem is over.