Have you gotten your boxer to the vet yet. If not please check his gums to see if they are a nice pink colour. Last October my Boston was acting the same and because I had my daughters dog here for a few days everyone thought he was having behavioural issues. Even my vet thought the same thing over a phone call visit. I didn’t and brought him in. Turned out the lethargy and not eating much was something called IMHA. Immune mediated hemolytic anemia. His body was attacking his red blood cells and killing them off. The vet looked at his gums and whipped him in the back right away for bloodwork and within a few minutes we knew what it was. If I hadn’t brought him in when I did I would have lost him and almost did anyway, He’s still being treated for this. I had never heard of this condition before and it scares me to think how close I came to losing him. I don’t want to scare you, I just wish someone else would have been able to tell me the possibility. I hope your issue is much easier to get over and hopefully it is just behavioural. Just good to get it checked. I hope to see a good update.