Hmm... In this case, I think you juz like a mature man who is
much older than you. It is normal at your age.
If a guy who likes a girl, after the first dating, he will
continually sms/call you or tries to have more dating with you.
There are only few reasons why he won't reply instantly :-
1) He is too busy to reply you (acceptable, but depends on what
kind of work/job)
2) He seldom use sms/call someone (??? 
, may be
he doesn't bring his hp always.... may be he turns off his hp
during working hours.....I have no comment)
3) He juz treats you as a cyber friend as he treats same way with
others online (depends on what is his priority for you in his
heart/life, so don't expect something)
4) He might has gf liao or he is with other girl when he receives
your sms(whether you like it or not, juz be careful)
p/s - Anyway, don't judge a book by its cover, try to know more
about him including his friends and his lifestyle (of course keep a
distance with him first before having closer relationship with
him)
Hope you will take more considerations before making any decision
for further development of relationship. Good luck 
