I work in a Kansas jail so my information may be anecdotal at best.
#1. I can't help you with this question.
#2. In my facility the money sent to inmates is held in an inmate trust fund. Any debts that the inmate incurs must be paid before the inmate can use the money for other purposes. Because this money is held by the facility, and the facility prohibits inmates to possess any form of tobacco product...or "drugs" of course, they would not be able to use it to purchase these items directly. In fact, barring debts, the money may only be spent on an approved commissary menu. Not to say that it's impossible to trade these commissary items purchased for tobacco or drugs (contraband items from other inmates).
#3. It's not necessarily a line. I am the commissary agent for my facility. Despite the three squares the inmates receive daily, I have seen inmates spend hundreds of dollars on commissary items. Things like ramen noodles, candy bars, chips...you name it. Usually if it can be found in the snack food isle of your local gas station, it's on the menu (these items 'keep' during shipping). Some guys just don't think the meals are enough fill them up. But we're not trying to fill them up. We're feeding them. There's a big difference.
And another possibility (but because the dollar amount is so low I find it doubtful) is that he's trying to repay a debt to another inmate.
What I would do is ask your son to send you a copy of the invoice for the $15.00 dentist bill. Then contact the facility and ask them how to make arrangements to pay it off directly.
All this an anything else is a mom's decision to make.
God bless,
S.M. Mahan