Hi Anniina,
I'm not seeing the point of you telling him any such thing. It's not like you have to walk around with a fact sheet around your neck, listing basic metrics like shoe size and number of times you've had sex.
Where it gets a little dicey is if you two were "together" at the time, but you weren't. You were still trying to figure out if you want to be together or not.
If you want to have a relationship with him, then sitting down to tell him you had sex with another guy is not the way to do it, just because naturally we all tend to be somewhat jealous. Now, honesty is always the best policy, but I'm just not sure it's really relevant here, certainly not at this stage in your not-quite-a-relationship. It was right after you asked him where your relationship was, and you ended up telling him you're going to see other people.
On that topic... if you're wondering if someone likes you, watch what they do, not what they say. People always try to put labels on your relationship, asking if he likes you, what he wants to do later on, etc. Guys and girls both do this, and it makes people feel boxed in. The human mating ritual is far too complex for those types of categorizations. It just "sort of happens".
You want to be together? Get together, do something together, have fun, and as you have fun and laugh together and spend time together, your relationship will blossom. Again, no labels. At one point you'll both think it's sill if you weren't "exclusive" so you'll go exclusive and agree to not date other people.
And so on. Make sure to use protection (a condom) when you have sex (yeah, we say that a lot), especially since you had sex with someone else (hopefully with protection).