They arrested him, and he's in jail now. What can happen to him?

They will hold him till he has a probation hearing. It may be a week or it could be 2 months or so, some areas are faster at hearings than others.

At the hearing, he may be released and put back on probation, he may be given a short jail time additional to serve, or given more serous prison time. It is completely up to the judge.

Here in Ohio we recently had a case where the lifetime sex offender did not register and ended up doing some 6 months of jail time.