I am no expert, that's for sure. But I have come to the realization that all must be comfortable for things to work. If you are not comfortable, that discomfort can lead to lots of negative emotions and problems later. I say keep talking and negotiating until you both (or all three) find common ground that you all are comfortable with.
I do know that feeling excluded is not good, so my advice is to talk, talk, and talk....and do not settle. Does he know you are afraid of exclusion? Have you thought about time with Dennis yourself separate from your partner too. Have you asked your partner how he would feel if you had a connection with Dennis, as well, that is separate from him in the same way your partner is trying to establish his connection with Dennis?
I hope this helps.....