I don't think you are overreacting if he's looking for relationships and pretending to be single.
This being said, how old was his profile on the dating site? Could it just be an old one he forgot about from when he was indeed single? Or maybe he started it, and there was no option for polyamory so he wrote "single" but expected to tell people the truth as soon as he could (did he describe his relationship at all in the text parts of his profile?)
I'm more worried about the fact that if he started the profile recently, he didn't tell you anything about it. If he's already going behind your back, it seems more likely that he would be lying to prospective partners as well.
But maybe it was just a liberating gesture for him, and meant nothing. Maybe he never talked to anyone there. I think you should tell him you found his profile and ask about it. See what the explanation is, and make sure to stress you want honesty between the two of you and with other people.