I actually disagree with most on here. Its out of character for him not to message you back. And OP didn't say she needs a reply immediately, but it was 5 hours later when he did respond with no explanation and just saying he is tired. Of course thats going to raise a little concern if its not something that usualy happens. especially gievn OPs background of her last relationship.
It doesn't mean you shouldn't be in a relationship, OP, or that you are oddly clingly or whatever. when you see him, ask how his evening was and hopefully he'll give you a normal answer like any normal person would - went out for drinks after work, lost track of time, whatever.
In an exclusive relationship its not odd to want to know your partner is ok, no matter how long that relationship has been going on for. Op is not demanding he tells her where he is every second, but this 5 hour dissapearance when normally he'd let her know if out of character. Some people need a little more reassurance than others, especially after a relationship where they were cheated on or dismissed. Thats something OP needs to work on, but also he needs to take into account if he wants to be in a realtionship with OP. It does not mean OP is controlling or clingy, it just means she needs to be with someone who is willing to help her with this a little, not someone who just does what they want , when they want and refuses to acknowledge thats not ok for their partner at times. Some people are fine with this, others aren't. Neither are wrong.
I'd just wait and see OP. Might be he is a bit flaky or hadn't considered you'd be worrying. Just tell him you were a bit concerned you hadn't heard from him and leave it there. Then he knows and if he continues to make a habit of it then its up to you whether this is someone you want to be with. A quick text takes seconds. If he is really wanting to make this work with you he will consider this minor concern of yours and reassure you.