If you're able to talk to your ex, maybe sit down & come to an agreement on how much time your son can hang out there and let HIM create the boundaries. Let him know that it is important to maintain stability with your son & at the moment, he is acting up. If you make it about "let's work together to do the best for our son", then how can he argue... unless he has other ideas of what is best. Then you can counter with the problems it's creating... behavior or grades, etc. Do not bring up how it affects you or the other kids because it needs to be about his son... he doesn't have to care about how it affects anyone else.

How old is your son? I think my opinion would differ greatly based on his age....I think if he is in his mid-teens or older (14+), I would think that it's really sort of our of your control as long as homework / chores are being done, grades have not slipped, and any outside activities he's involved in have not suffered.

I would try to look at the benefits that he will achieve emotionally by reconnecting with his dad, instead of just looking at the impact on you....and I agree, the newness will of course wear off, unless the dad has or allows him to do things that you do not...e.g. video games, junk food, a dirtbike...stuff like that. If that is the case, then by all means you need to talk to him and come to some agreements on what is allowed and what is not.

I take this approach partially because I am seeing the effect it is having on my GF's 12 year old son that his father moved 1000+ miles away from him for a girl he met...the kid is crushed...and it's not exactly like he was a role model of a dad to begin with...but he is HIS dad...and I know if he ever moves back, my GF's son would want to spend as much time as possible with him (for a while, anyway).

Several things I'd want to be sure of for example 1) the son can't go there when no one is at home (yes, I know he's a teen) 2) a calendar goes up and all family plans are scheduled in advance so son knows when he has to be doing something (like a family event)...also allowing Dad to request in advance something family wise going on on that side of the family. Everybody will know when a certain day if off limits.

What kind of schedule does Dad have for work? If dad is off all weekends, and there are no plans, why could not the father spend a day with Dad doing father/son stuff? The set and required issue there would be that he can't leave home until whatever weekend household chores are done and any weekend homework/school projects are finished.

Is Dad capable of assuring his son do some homework at Dad's house? There might be school projects Dad could help him with at Dad's off. The idea I think should be that the homework is getting done and grades don't suffer no matter where son actually does the work/project.

If the guy is trying to establish a relationship with his son, I would not discourage it. Unless there are issues that develop that (as MKroopy suggested, or worse such as drinking, deterimental influences ect) I'd let the son see his father. The novelty of seeing Dad every day I think will get old and there will be the time coming where son wants to spend time with friends and not either parent.

If Ex and you communicate well and can come to guidelines and agree to the whats, whens, ifs you should be able to work it out. Dad has to back you up though that if son does not do his homework, grades suffer, ect it will be time to re-discuss.

I'd make it real clear to Ex (and son) that you will not tolerate son running back and forth whining from one to the other when one of the parents p*sses son off. Just becasue son gets angry over something at home, he can't stomp out of the house and run hide at Dad's. If the open visitations aren't working and/or pitting parent against parent becomes an issue, you'll have no choice but to put an official parenting/visitation plan in order.