You sound like a great step mom. My ex is a much better dad because of his wife's influence, and I do think that you are the only one who can really bring this up with your husband, because coming from his ex wife, it will just sound like nagging to him.
I would recommend talking to your husband about the issue and saying how much his son needs HIM right now- not another buddy, like your son, not even his step dad, although if they get along, that's great. He needs and misses his DAD and as the grown-up, your husband needs to understand this and take charge.
Suggest some things that they can go and do, just the two of them together for the next time your step son is around. Does your husband fish? Fishing is a great and quiet way for some 'bonding' to take place. Or maybe just go play catch or for a bike ride or a hike- anything where it enables them to hang out and also TALK to each other.
If there are specific concerns about your step son's behavior, from either you or his 'other' family, is your husband aware of them? Make sure he understands that YOU are also concerned about them, so he doesn't just think his ex is being difficult, and so he cannot blame it on the stepdad, etc. He needs to know that there are real concerns and that ALL of you feel the same about it.
My fiance, my ex and his wife and I have all met more than once with a professional mediator to work out specific issues and parts of our parenting agreement. It is cheaper than going to a lawyer over things ( we split the cost) and it really lets everyone speak their minds, but with an unbiased person to keep things civil. It has helped us all A LOT to get us on the same page on different issues.
It's so important that the adults in this boy's life present a united front when it comes to his behavior and well-being. But his dad needs to realize that he needs to step up and be responsible. Good luck!!