Do you think he doesn't wish to stay in touch?

My partner had a rough relationship with his dad when he was growing up. He abused him mentally and physically. Of course when I met my partner, I suggest that he make peace with his father because the bitterness was unhealthy and it made him become someone he's not, if you understand what I mean. So he slowly reach out to his dad and I guess out of loneliness, his father tried to have a relationship with him again. He is a much better man now and is slowly learning to be a father again. However, when we moved out of state, he slowly lose touch with us. My partner calls him frequently at first, but the conversation was always the same and there wasn't much to talk about. Now, the only contact we have with him are festive cards, birthday cards and gifts. When he sent me some money for my birthday, I emailed him to acknowledge that I have received the gift and thanked him for remembering my birthday. I also asked how he's doing and all that, trying to connect with him. It's been over a month now, and I still haven't got any reply from him. My partner is starting to feel the bitterness again, as he feels his efforts are very one sided.

We have tried emailing him, but he hardly replied, if he did, it was a very short and brief one. When we called him, we'd talk for less than 2 minutes and he'd say he has to go because his phone battery is low/ incoming call/ not feeling well, need to sleep soon/ somebody at the door etc etc. What's your opinion on his behavior? Is he avoiding us?
He has another child, and their relationship is totally different compared to my partner's. When they talk, it would last forever, and if he needs help, he'd go all the way for her, being the best dad in the world for him.