My son was doing really good in school up to fourth grade, for some reason he lost interest in school and now that he is in seventh grade, his grades are very low and lost interest in school. I tried grounding him and it did not work, I talked to him and it did not work, and I do not know what else to do. Is there any suggestions that I can use to help him?

First be sure he does not have a learning problem,
Next sadly schools often move kids ahead and not hold them back if they are not preforming well to the point the kids can not do the work.

So perhaps have his education level accessed by independent person.

A tutor may help and if possible even a change to a private school if you can afford it.

Do you work with him on the harder subjects for a period each evening ?

Is it a matter of him being capable of doing the work, and just choosing not to, or does he actually struggle with it?

What does he say when you talk to him about it? How does he do when you go over his homework? What do his teachers say? What sort of comments have your received on report cards and at teacher conferences over the last few years?

Any changes in the family... moves, divorce, deaths, etc.