first of all, i'm really sorry you feel like this and i'm sorry it happened. i've been through something similar and it can really mess with your head. so, the definition of sexual assault is when someone either touches another person in a sexual manner without consent or makes another person touch them in a sexual manner without consent. that includes unwanted kissing, touching intimate parts of someones body even when they say no. the legal definition of 'sexual' depends on whether a 'reasonable person' would consider something sexual (according to the cps.gov.uk website), so i guess that specific part is up to whether you think what he did was inherently sexual (though disclaimer: it was).
you said you needed to go and you didn't want to do anything with him at that specific moment, but he didn't listen to you. some may say "but you didn't say no", you didn't say yes either. and anything other than a yes, (or the equivalent) is an automatic no. he couldn't respect your choices and the things you needed to attend to, and tbh he couldn't control himself to point where he came on your JEANS.
i do understand why you are confused though, especially knowing you kissed him back and he apologised afterwards. i don't know if i should label anything definitively, but he definitely crossed a line, and that's not okay. also, did he apologise because you brought it up to him (if u even did), or was it a case of him just apologising afterwards because he realised what he did was not okay? honestly, i'm really sorry this happened, and i'm sorry your friend wasn't able to recognise boundaries and control himself.
keep yourself safe (n drink water <3 )