Re: suggestions needed
That’s what I would have suggested too. He should make it up when he comes home. It’s also good to know that it is possible for kids to make salaat in school. We did that starting in high school, and I’m sure we would have been able to do the same while we were in elementary school. You are right, you can just ask the administration. If you know of any other Muslim students in the same school, try to get them (and their parents) involved as well.
Aside from making him make his salat, I would also recommend starting taleem (Faza’il e Amal) at home so that you can keep the importance of making salat in front of him from this young age. Also, try to take it up a level and encourage him to make salaat in the masjid whenever it’s possible.