The general rule for changing from a snacker to a nurser is to gradually increase the time between feeds. If he's wanting to snack every 45 minutes, you make him wait a full hour for each feed for a couple of days. Yes, he might cry. But, 15 minutes of crying won't hurt a baby, especially if you're comforting him. And you're only letting him cry because you're doing what's best for him in the long run. He needs to take good feeds and good naps in order to grow properly. Then you move on to making him wait for an hour and 15 mintues for a couple of days. Then you move on to making him wait an hour and a half for a couple of days, and so on until you get to a regular pattern of 2.5 to 3 hours between feeds.
But this is the general rule. If you could give us a few more details, we might be able to address your situation more precisely.