Should I give up??

My cousin and I use to be really close. We were inseparable when we were kids. We grew a little apart as we got to junior high and high school, but still stayed close and friends. We have been graduated for 3 years now. He joined the marines straight out of high school. He is almost done with his time in the marines. I have tried to keep in contact with him and I know he is busy, but he doesn’t even open my messages anymore. I’ve never received a letter back. I continue to try to talk to him because I love him like a brother. The last time he answered me was a year ago when I told him happy birthday and he said thanks. He’s never answered since or even opened a message on messenger since.

Should I just give up trying and take a hint or should I keep trying?