Sorry to hear MrsMharket. Onwards and upwards tho!
SingleMum01. From what you've said I hear some alarm bells ringing too.
Before I met my now GF, I used to frequent dating sites. Knowing how they work, if he's only messaging you then either he is clueless or lying. In my experience the odds of getting a reply are about 1/10. Out of that 1 contact, the odds of getting a date is about 1/4. So if he's got half a brain cell he won't be just talking to one.
Of course he could be nothing like me & some of my friends who used the machine gun tactic.
However, if he's only messaging you from work and not answering calls/texts in the evening, that's another alarm bell.
What I would suggest is that you go on your date but just take care. Certainly don't progress anything until you know either exactly where he lives, his landline number or something which confirms he's single and not playing away from home. You can do this for example after the first date by calling him up for a conversation and then saying "oh can I call you on a landline, its cheaper".
Lastly but this smacks a little of desperation at the moment, you could set up another profile on the same dating site, use someone else's photo and drop him a message. See if he replies.