Hi, Mom, and welcome.
I'm not a doctor, so this is just opinion. First, I'm presuming your future son in law is an adult, so you may not be able to force him to do anything. He may be going through some withdrawal symptoms, depending on what meds he's been taking. The most you might be able to do is call the pharmacy or his doctor, but you likely won't be able to get anywhere without his permission. Have you told him how concerned you are? Ask if he wants you to help in some way. If he says no, my guess is you may have to have him wait until the pain returns and then my guess is he'll change his mind.
The only caution I could think of - but again, I'm not an doc! - is whether any withdrawal may be life threatening. If you know what meds he was taking, you could call a pharmacy and ask in general if a quick withdrawal from those meds could be life threatening. If so, then you may want to get some medical help.