I wouldn't worry too much if your son leaves the bathroom door open. One of them will probably just hide, possibly the older cat.

First order of business - FELIWAY SPRAY!!!! Spray the room. Leave the carrier in the middle so they can both see and smell each other. Wipe towels down, one for each to smell. There will be hissing. That's normal. Put new kitty in bathroom. They'll smell each other under the door and play footsy.

I have two baby gates I use stacked in the doorway. Eventually, you just take down the gates, keep a spray bottle of water and a spray can of Feliway handy AT ALL TIMES!!!! As long as there's no blood shed, just let them work it out. Kittens are far easier to incorporate into a household than two older cats.

Always feed them together, as it will remind them of a happy experience. Food is a wonderful motivator. That's what brought out Lucy and Puff.