What kind of testing products are you using...strip or liq reagent?
Do you have live plants, if so, what kind and how many
I ask because with the numbers posted...your tank is not cycled unless you have lots of live actively growing plants...also, what is the pH and water temp
On to the Betta...normally the Betta do not lay on their side at the bottom of the tank unless something is wrong...with that said....this species being domesticated they can do some odd things at times...thats why I said normally...not never or always.....in a 5g filtered tank with your water change schedule of 40% weekly should maintain water quality, however, depending on the cycling stage and I am not sure on that since you are not showing any nitrate, but this could be due to live plants if you have them......you may need to add a 50% water only until you are fully cycled...
He is eating well and responding to you..thats good, I am assuming that filtration is not too strong causing him to tire himself and needing to rest, water temp is okay and he has items in the tank toward the surface to rest on if needed, he not over 2 years old.
Although he is resting on his side he swims normally and is not having any buoyancy issues....correct.....