okay.. here's my 2 cents
you can use a smaller mouthpiece .. like a 10 1/2C .. that MIGHT help
one way to build up chops and tone is through the use of staccato attacks and then stacatto attacks on intervals.
They have to really be stacatto and really attacked..
but the kid has to put in 30 minutes a day ( at least).
random thought here ...Did you ever get the slide free?
okay .. I would make up a cd of the exercises since he really isn't going to have to read music..
The cd should have something fun for warm ups and then something of a fun track to play along with.
If you don't have the means to do that shoot me a PM .. maybe I can help.