If it's part time employment, he is still considered available for work on the week he's not in FAS and can sign on for that week. If he gets the job, he can tell SW about his new work pattern (one week on, one week off) and they'll advise him further.
Regarding the medical card, if your father has been on JA for a year or more, he doesn't lose the medical card immediately.
If the work is part time, he can keep the card for 3 years after his wages first go over the income guidelines.
If the work is full time, he can keep the card for 3 years from the date he starts work.
I'm not 100% sure on college grants, but I think that if you've been assessed for this year, you get the grant even if circumstances change - but it may affect grants for next year if he was still working. The local authority who issue the grants will be able to tell you for certain.