My cat has stopped nursing her kitten too early?

I think she's gone into heat. The kitten (yes just one) is only five weeks, he's old enough to be trying solid foods but isn't. The mom gets up whenever he tries to drink. So I've been feeding him from a bottle with pet store kitten milk for a few days, and he gets a lot down, but I have a full time job and I'm afraid of malnourishing him. I also bought the second stage kitten food that you mix with hot water like gruel. Does this have less nutrients? How does a kitten go from eating mushy baby food to crunchy nuggets? I also fear once I start the gruely stuff, he'll need it every four hours or something like that. What do you think?