The chewing thing is actually pretty normal, at 8 months they've gotten so used to the breast that the bottle feels strange, so they explore it first before committing. Key thing is to let someone else do the bottle feeds while you leave the room, because babies can smell you and will just hold out for the real thing. Also try offering it when he's calm and a little drowsy, not when he's already worked up and hungry, that's usually when it clicks faster. Between the Hegen and Avent, some mums swear by Hegen for latch similarity, but honestly just keep trying both and see which one he settles with.