Tough situation. My son is also a meth addict. He has a five year old son who comes to spend one weekend a month with my husband and I. I use to let my son know when his son was going to be here but in all honesty.....I would be setting myself up with an expectation of my son being there and being a father. It seldom happened that way. He loves his son but in active addiction, he is not really capable of being a parent. I don't mean that as a judgement of him. It just is what it is.
I also realized that I was putting myself in the middle of everything. In between my son and his son. In between my son and his exgf. It seemed to me that I was trying to keep everything "together" and it's not my job to do that.
I'm now enjoying the time I get with my grandson, I don't inform his father (and by the way, his father doesn't really ask either) and I make the visits about my grandson and us (his grandparents). Not about whether he's going to see his daddy or not.
One thing I have to agree with others about......my son's relationship with his son will not make him use or make him stop......unless it becomes my son's decision and his priority. My involvement, allowing him to see his son, disallowing him to see his son....none of that really makes a difference until the addict decides it does.
An entire summer is a tough situation. Children always add a difficult dimension to any situation involving addiction.
gentle hugs
ke