Hi. Welcome. I am a Licensed Master Social Worker with more than 20 years experience working with individuals and families. I am also a mom of Twins.
You are doing great with what you have done with him around this behavior and great that you have identified the trigger of his being tired.
First let me say that this is not uncommon for children his age to hit.
They don't have the language skills to express what they are feeling and don't have the emotional skills yet to deal with being tired, hungry, frustrated, angry, etc.
So they use hitting as a way to release those frustrations. That is not to say that it should be accepted and you are truly doing a great job with how you are handling it.
Keep your explanations short and simple. "Hitting hurts. We don't hit."
and because you know the trigger such as being tired or overtired then try and create the environment to minimize his emotions when he reaches that tired space.
If he still naps then make sure you aren't missing that window of getting him in BEFORE he reaches that space of being overtired.
and if this happens later in the day, try moving things up by 15 minutes such as dinner 15 minutes earlier, bath and then in bed so that you are catching it before he hits the wall of exhaustion.
you may also try that when he sits you to disengage. say the simple statement of hitting hurts. we don't hit. and then turn and walk away.
*hits..not sits...sorry for the typo.
The time outs are effective for that moment, but at his age they may not have much staying power in the sense that he will most likely do it again.
The good news is you know the triggers, so trying to avoid them is helpful and the other wonderful thing is that his behavior is normal and he will outgrow it and outgrow it well with your love, patience and guidance.
I will wait for you to come online so we can have a dialog.