I hope at the very least SCOTUS will side with this businessman. Ruling in favor of forcing speech would have very bad ramifications far beyond business transactions.
The SCOTUS, of course, is addressing the Colorado baker's situation, not the T-shirt maker. The T-shirt maker should be on firm ground although that, of course, doesn't prevent an angry customer from suing and forcing the T-shirt maker to incur legal bills.
I suspect the SCOTUS thinks very carefully before accepting a case, and its agreement to accept Masterpiece Cakeshop strikes me as ominous. Bad facts often lead to bad law, and the SCOTUS likely sees an opportunity to bolster the law against discrimination in public accommodations. The case was shot down on freedom of speech grounds by the lower court (the grounds that would have supported the T-shirt maker), and is now before SCOTUS on the retooled and very shaky ground of freedom of religion. There is plenty of precedent - including Supreme Court precedent from the sixties, I believe - for the proposition that a claim of religious liberty doesn't allow a business to discriminate. (The sixties case, if I recall, involved a Bar-B-Q restaurant that claimed it didn't have to serve black customers because of the owner's religious beliefs. That claim was shot down so fast it would make one's head spin!)
Bottom line - the Colorado baker screwed up by rejecting service before even inquiring what the customers wanted on the cake. He's going to lose, I predict. The proper ground is freedom of speech, not freedom of religion. The T-shirt maker has the law on his side, the baker, not so much.