My boyfriend is on probation. He went to jail for probation violation and they reinstated him. Now he is in jail again for probation violation but he has no new charge. Could he be reinstated again?

Yes he could. But with each new violation or arrest, they can revoke him and give him a jail sentence. It's all up to the judge but he will take his previous violation into account along with his current one. He's not going to go too easy on him. He'll be lucky to still have probation.