I think it would be good for you to apologise and explain to him what you've explained to us, but in a totally neutral way.
Take the angle of "we've got a mismatch here, so how do we go about finding a solution, together?".
Don't put pressure on him to feel bad or make all the concessions, but don't do that yourself, either.
Any SN aside, 13-year-old is easily old enough to understand the idea that some people need time alone, and that that doesn't mean they don't care. What shows you care is how you handle it, within any other limitations.
The best ways of showing the other person respect over this is to be honest about where you stand - rather than expecting them to guess when you're not up for a chat and why - and to bring them in on finding a way forward.
Everyone would have benefitted if my mother had taken that approach, instead of putting pressure on herself to be available the whole time. Because she simply couldn't be available the whole time. So she'd often respond inappropriately, because her social self wasn't "online". If she'd admitted that, I'd have felt safer trying to interact with her.
But instead, she insisted that it was always ok for me to approach. Which, as a child, did nothing but make me feel like I'd done something wrong when she reacted strangely or aggressively. If she'd said she needed time alone except for emergencies, but that that didn't mean she didn't care, and told me when, I would have understood that perfectly well, from age 7 at the latest.
I think by pretending she could always be mentally/emotionally there for me, she was trying to make me feel safe, but instead, the unpredictability made me feel very unsafe.