I admire your concern for your stepson. I too and a stepmom and know how challenging it can be. I would have dad try to reason with the grandparents and work out an alternate solutuion to help make up the loss of time. Offer to come pick him up from them and bring him back after his game.... it could be that they are not "up to" being out in the weather. Yes, it would be extra work for you and hubby, but afterall, you would be doing it for your stepson. But, most importantly, make sure your stepson really does want to play. Make sure the grandparents aren't telling him he has to choose (them or football) and applying guilt to him. Maybe have a sit down with the parents, the grandparents, and the stepson.... encourage the stepson to speak freely about what he wants. Make sure he knows that it is ok to want something differently than what someone else wants for you. If this doesn't work, I would seek a court order altering the visitation or something. I am not sure how old your stepson is, but at some point the court can appoint him an attorney so that he can actually say what he desires..... and I believe that age is 12 or 13. Not sure.... my daughter is 13 and has told me she doesn't want to have to go to a judge, that she is happy the way things are.... I told her she doesn't have to..... it's only for when you want something to change and your parents aren't in agreement.
I wish you good luck. Being a stepmom brings about a whole different set of challenges and rewards. Keep up the good work!