
Hi Claire, I think you have to establish with your dog that the toy is only fun because the toy is an extension of you - ie. the toy (be it kong, frisbee, stuffed animal) loses it's importance but it is you and you alone who make it fun. That's why he should not have free, unsupervised access to any toys - which he must have if your younger dog is able to "help himself" to those around ;-)
Your aim is to condition him that it matters not what toy is available to be played with (or stolen) from other sources - those toys are of no use or interest to him because is real desire is to play with you - and you just happen to choose to play with a toy :-)
The alternative is to remove all toys - permanently. Give him bones/chews/nylabones whatever as substitute but try and desensitise his interest in toys and keep your own play and training sessions with him as treat based only. This does seems a shame tho' :-(
It may also be worth thinking about whether this specific dog at training is somehow upsetting your dog in a way that does not involve the toy. There may be subtle doggy signals being given off that are causing your dog to behave in this manner.
Good luck, Teri :-)