I have a long distance relationship and I basically set a time for my girl to message with her, say 8pm to 10pm, or whatever suits you. Often during the day it is not convenient, people have work, study, want to rest, have a bath, want to eat, watch something, listen to music, relax, send of important emails, return important phone calls, etc, etc.
My girl has said that she wants to message more during the day on a random basis but this is rarely convenient. She tends to message a lot back & forth and unless she's happy with a long wait between messages or me forgetting to message back it's just not practical. Personally I prefer to have a dedicated time in the late evening/night when we can message back and forth uninterrupted and isn't constantly messing with tasks during the day. The old adage of women being able to multitask men not so happy too comes in here I think.
Remember also that the guy needs to message other family & friends. Also everyone needs time out doing what they do to relax whether on games, forums, etc. If you want his attention you could send him some sexy pics but again it's not going to be easy for him to get into it while at work, sat with family, etc as it's a while different ball game.