So, why don't you say to him "yes, you can pick up your plane parts but you need to take all of your stuff with them". I don't think there is anything wrong with that at all. Just tell him to.
And look at the him taking his time with the model as a good thing. It's a good healthy hobby. GAL is healthy and he is choosing it in a healthy fashion. I know it's hard to think of it that way. He could be GAL by drinking every night and dating a whole bunch of chicks, but he is building a model airplane. That's a good thing.
