Can my boyfriend sue me for the money he put into remodeling the inside of my garage?

Unless there was an agreement to pay him the answer is no. He can sue you but will not win.

The short answer is that he can sue you, but his likelihood of success are somewhat slim. Assuming that the two of you do not have a written agreement that states that if he puts the money into the garage, he will get something in return, typically an ownership interest, all your ex has to rely on is the theory that an oral contract was entered into between the two of you. Your ex would argue that he put the money in with an understanding that he would get something back.

While he can sue, he will not prevail. He can claim that he used hi own money for the remodeling, he paid no rent. If he was on his own, he would have. Thus, he earned a benefit, which he paid for by the remodeling.