Abilify is an anti-psychotic, it is widely used off label so best to ask what he's prescribing it for yes. Could be a sleep aid or he could be considering it another MS and wants to add it as back up for the Lamictal.

Best is to ask, I personally wouldn't be comfortable taking it if I wasn't some kind of psychotic, but many people are more open minded about such things.

I think its lovely that you and your mom have such a good relationship, its great when we can ask the people closest to us for feedback. Sometimes we need an outsiders perspective, one we can trust, and I'm glad you have that.