Is he drooling saliva or formula? Sometimes babies who were exclusively breastfed take longer to get the hang of a bottle nipple, even it if is the slowest stage. How long does it take him to eat the whole bottle? If it's a long time, he may actually be bored with it because it comes out too slow! He is old enough that that may be the case.
If not, watch him while he is eating and look to see if he is sucking really hard to get it out. If so, he may need a faster nipple. I know that sounds contradictory, but it's tough to figure out and sometimes what works is the thing that doesn't make sense! :)
I used the Avent bottles with my son, and he did well. I BF exclusively for 6 months, although he did get bottles of breast milk starting at 3 months. I liked them a lot, but it is a trial and error sometimes. They do have a bottle, and I can't remember what it's called, that is supposed to be "just like" the breast, but my son got very annoyed with it. Anyway, I hope this helps a little. Good luck to you and your little droolbug! :)