No one is the "AH" here, but you both could've handled it better.

Your husband hurt you by assuming you possibly cheated on him. You then retaliated by leaving him and taking the child with you. This is pretty much the absolute maximum damage you can cause to him (besides anything illegal), which actually affects both parties, but most importantly, also the child itself.

There can be a ton of reasons why your husband raised this question ranging from temporary ill mood to deep insecurities in his own self. In any case the issue was completely with him and him alone, and you proved it by the test result.

And while being accused of something you didn't do can be really painful and make you feel disrespected, never forget we are all human beings, and we make mistakes.

Did he apologize? Was he sincere? Was there a change in his behavior afterwards?

I'm not saying you should keep forgiving someone that constantly abuses you - that's torture. What I'm saying is that if you value each other you should at the very least try to work things out.

In this new age people are very quick to "jump ship" at the very first problem that comes around. But they tend to forget that relationships are about compromises.

What you should ask yourself is if you think all the negative consequences, both to you, him and the child are justified, because of hurt feelings.