I was very suggestible after the injury I sustained too. It slowly started to go away - I am still more suggestible than I was, but I try yo watch out for it. Before the injury I sustained I wasn't suggestible at all - quite the opposite! It's been interesting to see what it's like to be suggestible, but also somewhat scary. In my position at work, suggestibility is not a good thing, so I'm lucky that it's nowhere near as bad as it was.
I returned to work a little over a year after the injury I sustained. I think if your son thinks he's ready, then you should let him go. If he doesn't do well, they'll just send him home. I work in a somewhat dangerous field and I didn't think I would be able to return because I didn't have the wherewithal to pay attention enough to not get hurt, but you know what they did for me? They put me on "light duties". I didn't have to do the all the same things that I did before the accident - I only had to do things that I could do. I think some employers are even able to give people completely different duties until people get better. And returning to work helped lift my mood a GREAT deal - which I'm sure has helped me to recover a great deal. I was so anxious and worried I'd lose my job before I was able to return. AND I believe it's been therapeutic and rehabilitative for me. Everyone at work agrees too. I'm doing much better now than I was when I first started working. I'm even able to drive myself to and from now - I had to get rides to and from for the first four months. I started working 12 hours a week, four hours MWF. Before the accident I worked a 4-10 week, 4 days of 10 hours each day at this job and 15-30 hours every weekend at my other jobs. I was able to increase my hours to 20 hours a week in Dec. My FMLA ran out, but everyone can see the progress I'm making and the FMLA doesn't seem to matter, especially since it looks like I'll get all better - everyone at my work is rooting for me. And I'm able to feel even a smidge productive and that makes me feel better too.
If your son is taking FMLA - it's only 12 weeks of protection. If it's important for him to have that, and he takes all 12 weeks off completely, then when he returns he'll have to work 40 hours a week. Or he could take 6 weeks off and then work 12 weeks part time and be able to build up his strength better before he works full time again.
I think if your son feels ready to return and his Dr. thinks it's ok, then you should support him to go back. He might fail and have to return to resting at home, or it might speed up his recovery like returning to work has done for me. You'd be surprised what being required to think about things at work does for reconnecting neuronal pathways.