DS drinking less... is this normal behavior?

DS will be 5 months in a few days and it seems like he's not drinking as much formula.
Around 3.5 months, he was averaging 28 oz a day and he continued that amount till about 2 weeks ago.

I was gonna start increasing his formula intake from 6oz to 8oz per feeding because I was afraid that he wasn't getting enough. However, he started to push the bottle away after only having 3-4 oz, as if he's had enough. I'd try to feed from different positions and angles or wait a few minutes to see if he'd take more; sometimes he'd finish the bottle but sometime we'd stop because he just won't have it (and I don't want him to develop eating disorder or something).

So, is this a normal behavior?
Another thing, I think he might be teething, could that have something to do with it?

Teething babies can opt out of feeds because of pain from the suction, definitely. You could try massaging his gums or using a teething gel - we used Orajel Naturals brand with my daughter because I had a paranoia about -caine anesthetics ... and Hyland's has belladonna in it, which I found troubling for my heart-defect baby.

A breastfed baby could also pull a nursing strike at around this age - they're starting to get more interested in the world at 4 and 5 months old, you know!

If the teething gel, or a frozen sock (really - soak a clean sock in water, wring it out a bit, then toss it in the freezer 'til it's frozen, give to baby and VOILA! teething toy for practically free; you can also use a knotted washcloth), don't help, try burping him more often during a feed. He might be swallowing more air.

How often are you offering feeds? You may be able to extend the time between if you offer bigger bottles.

I agree with PP. I will also mention that neither of my boys (both BF, but bottle-fed when I was at work) took more than 6 oz in a single bottle feeding. I suspect their AM nursing session may have been a bit more than 6 oz at times, but I'm pretty sure that other session were around 6 oz.

Anyway, they hit 3-4 mo started taking 6 oz/feeding session, then never increased from that. So I wouldn't be overly concerned.