He is not sleeping on his side today. I know the water isn't warm enough I wrap a thermal blanket around tank till new heater comes (everything is curbside or delivery). I do treat the water and I'll get some strips. I have NO idea how old he is. I took it off a friend who got it from pet smart. I usually try replace water 2 months and deep clean every other month.
Okay- I am just going to lay out the info I know about bettas on the table.
Parameters:
pH: somewhere between 6.8-7.2
ammonia: 0ppm
nitrites: 0ppm
nitrates: 5-20ppm
temp: 78-82 f
Tank Mates:
Usually, bettas don't need anyone to be around them. However, if he showing lack of aggression towards the tank mates could be a good or bad sign. He could be so sick he has no energy to protect his environment or he could just be calm and collected.
Decor:
To help his mood, please put more live plants in the tank. Floating and planted will do him well. Bettas love floating plants and I have seen that the bigger the leaves the better, they will sit in them and rest.
What to do:
Test your water, please get the API Testing Kit. It is liquid testing. It is a bit expensive but its worth it in the long run, as well as cheaper.
Once you get your parameters, please tell us here on the forum. We can accurately help you from there. I do not see anything wrong with him from the pictures you provided. However, sleeping on his side is not okay, he could have some buoyancy issues (swim bladder malfunction, dropsy (unlikely), ammonia poisoning, etc).
Take some more pictures. The people on here can identify diseases such as ich or dropsy very easily.
If he is continuing to eat, he could just be adjusting in his new environment.