Your son's GP should have referred him to a neurologist: only neurologists are qualified to diagnose MS.
A lumbar puncture is actually not necessary to diagnose MS and it is certainly not a definitive test for MS, however it can speed things up.
However, it is really too early to be talking about MS. There are loads of mimics of MS (i.e. things that cause similar symptoms), some of which are temporary and/or fixable; in order to be diagnosed with MS, a patient has to have more than one attack; and it is also highly unusual for someone with MS to have "most of the symptoms" during their first attack.
So, my advice would be to wait to see what the MRI reveals, and then ask for a referral to a neurologist (or to whatever specialist is most sensible depending on the MRI results). MRI results shouldn't take too long - the images are available immediately; the delay is because a radiologist has to prepare a report and have it typed and posted.
If your son is terrified about having MS, it may be because he has some misconceptions. The truth is that MS reduces life span by somewhere between 4 and 7 years, depending on what you read and most people with MS do not end up in a wheelchair 24/7. In fact, most people with MS live quite normal lives: they study, they work, they get married, they have kids, they travel, they have friends, etc. It is not the end of the world.
Try not to panic. Take it one step at a time.