Manda, it is difficult to answer your question without more information.
It looks like your son is 8 months, correct?
Does your son have breastmilk at all, or only formula? Is he eating solids? Does he drink any other liquids? How long has he only wanted formula twice a day?
Every child is different, but I have a foster daughter very close in age to your son. She still is having bottles at least every four hours, though on occassion she will go a particularly long stretch, say 6 hours before she wants anymore. However, she isn't guzzling her bottles. 6 ounces is the maximum she will take at one time (she will make it *very* clear when she is done), and often it is more like 3-4 ounces. She does not typically take any other liquids. She does eat a small amount of solids.
I know the general guidelines about what is "typical" for a formula-fed only child around 6-9 months is to bottlenurse 3-5 times per day, consuming 24-32 ounces per day. I think my dfd may come up a little short of 24 ounces, but she is definitely eating frequently.