I would personally never ever take anyone out on the bike with me and I've purposely taken pillions off my insurance. I choose to take the risk and I won't allow anyone else to hand their life over to me, regardless of how much effort I put into my riding.
That said whilst I feel I am basically a competent rider, most people on here have far more experience than I do and will be far better riders. I would love to have the riding (and for that matter life) experience that you guys take for granted.
THAT said, I've seen a youtube video where a rider with his son as a pillion got barged (physically, with contact) to one side by a stupid ignorant bitch of a car driver who got all of 5 meters further on in traffic because of it and refused to stop for the collision, just giving him abuse through an open window. The traffic was moving at a fixed pace and if he'd been on his own no doubt he would have been over taking it. As he had his son on the back, he was leaving a very sensible gap (not a criticism) in front and holding back.
It's all about your ability, experience, the perceived risk and so on. Ultimately he isn't just your son - he is your wife's son as well and you must take her concerns into account. There might be a compromise to be reached or she might be totally opposed.
If you plan on doing it, my advice would be to invest in the proper kit for him, no matter the cost. I'd go the whole hog, including chest and back protector (if it were my son, it'd be a motocross style armour suit with a top of the range abrasion resistant jacket on top, combined with extremely well armoured boots, properly good and well fitting helmet as well as hip / knee armourer trousers. I might even consider a neck brace given he doesn't have to look around much) and literally no expense spared. I'd get him an airbag jacket as well. I could deal with a broken leg or arm or something (kids do that anyway) but anything more serious and I couldn't deal with it if it were my son and I were in charge of the bike.
EDIT: As I say, I have little life experience so I may be a little over cautious (I think the young often go one way or the other). I'm only 28 with no family / kids so it's all projection.
EDIT: or am I 29? Bloody morphine.
EDIT: nope, 28. 29 this year. Crikey.