On the one hand, it's such a valuable host body and brain. It's a huge thing to give up.
...On the other hand, he's just given away far more than that. From here, having Alloran or not isn't going to be a deciding factor in anything.
There's the part where probably he would have to give Alloran back within thirty days anyway, if 'voluntary hosts only' is the proposal they reached, or sooner, part of some other agreement.
Alloran hates him and will try to sabotage everything.
...Alloran is an Andalite, and a competent one, but he's not Leareth. He's nowhere near Leareth. And - the gamble he's already made, here, is that Leareth is on his side.
Eventually he goes back in. <I will do it. I - would like you to be present, so that the Andalites will know the conditions of it. We will need to sedate him briefly for it because otherwise he will definitely kill me, but he will be unharmed>