Q. Our RR 13-year-old grandson has lived with us for 15 months. His dad -- our son -- gives us $100 a month (his orthodontist bill is $82 a month). My husband will retire next year and I don't work.

Five years ago, the child's mother walked out after nine years of marriage and left our son with two children, a girl of 2 1/2 and the boy, who was 8. Our son remarried a woman with two boys, 14 and 8. She tried to be a mother to all the children for 2 1/2 years but then persuaded our son to give his children to his ex-wife. The ex-wife said she couldn't care for the children yet and asked if we would take them. What else could we do?

The children fought constantly; I cried every day and my usually calm husband got shingles. After nearly four months of this, I called their mother and said we couldn't keep both.

She now has custody of the girl. She gets $200 a month for child support, but has a terrible time making ends meet. She is a poor manager, parties a lot and tries to get someone to keep the girl every chance she gets. She takes her son every other weekend.

Our son takes both the boy and the girl one day every two weeks (no nights). He and his wife both work -- but he says he can't give us more money for his son.

Our present daughter-in-law will have nothing to do with us. Our daughter -- who was always very close to her brother -- is also completely alienated. None of us knows the reason for this and our son refuses to say.

I have begged him to take his son, but he won't, even though he still has custody. Having a teen-ager in our home is a great strain on us. We have an unusually good marriage, but this problem puts much stress on it. Our daughter and her husband have said they would take the boy if we can't continue but they have two small children and a very limited income.

We love the boy but he wasn't raised like our own children and it's hard to fit him in. He's a good boy, an average student and a good athlete. He must be disturbed by what has happened to him, but doesn't talk about it.

What can we do? We are being imposed upon, but we care a great deal about our grandson. We also care about our granddaughter, but she is out of our hands.

A. Some of the pain of divorce never goes away, but there is much more pain in your family than there needs to be.

Look at your son. He still hasn't come to terms with his divorce so he tries to detach himself from everything that reminds him of that first marriage -- including his children. He has put his emotions on hold. Your grandson also keeps his own counsel.

It's no use hoping for your son and his wife or for his ex-wife to take over. If they do it of their own accord, that's fine, but in the meantime, you and your husband are responsible for rearing him.

Your son, however, should pay you more than the $18 that's left after the orthodonture bill. If he won't, you might file for custody and let the judge set the payment.

In any case, you and your husband are surrogate parents and must treat your grandson as you treated your own children: setting limits, requiring good manners and depending on his help with family chores. You don't have to worry about fitting him into your household; like any child of any age, he's the one who has to do the fitting.

All of you need help to deal with each other in these circumstances, to accept the new pressures and to handle the anger you must feel toward your son. Ask your county mental health program for family therapy for you, your husband and your grandson, with occasional sessions to include your granddaughter and your own daughter to draw at least some of the family together.

You'll also profit by the excellent book All Grown Up & No Place to Go by David Elkind (Addison-Wesley, $8.95). It will help you help teen-agers in crisis, and all teen-agers.

This isn't all you need. Ask your daughter to invite the boy to her house for one or two weekends a month, or to drive him to some of his games or other activities so you and your husband can have some time together. In exchange, your grandson can do some sitting for them. The tighter he's woven into the family, the stronger and more secure he'll feel, especially if you tell him often that you love him.

Your unexpected responsibility is weighty, but the load will lighten as soon as you can accept it. And when it gets too heavy -- and sometimes it will -- take comfort. You're teaching your grandson that problems can be solved, if you try hard enough.