Honestly if you are not friends with him than don't pay much attention to him. There is nothing wrong IMO supporting against your rivals (especially when matches are in the same time) but logging in just so he could support your rivals that's just lame.
I have a similar story with a guy the did this always to me and I returned him the favor back
What angered me last season was one of "my friends" who I have supported his match way more than he has supported me for a lot of season. We were in the same league and despite him being out of the race he wanted to finish above me and supported the rival. I ended up winning but that left a bad taste in my mouth,.