maybe this will help you : dont worry about how MUCH your son is eating. he will eat when hungry, and stop when full. the first year is very confusing when keeping track of the amount he eats because for a while they will eat SO MUCH cuz they are growing, then they will eat next to NOTHING because they arent growing physically .... just relax, and be sure that this is NOT an issue, there shouldnt be a need to switch to a different nipple until he has frustration cuz he cant get enough out fast enough. a bigger nipple can cause him to "drown" because of getting formula too fast, and that causes gas and other sorts of problems.
with a bottle, this is probably far off, but remember to try not to let him fall asleep with the bottle in his mouth, OR if he does, remember to remove the bottle as soon as possible. even if he doesnt have teeth yet, you dont want to foster a habit that causes extreme tooth decay. a pacifier is a healthy, safe, and protective way for him to suck (a NEED of all babies) and not hurt his teeth.
usually eating too much formula wouldnt cause diarrhea... at least i dont think. im not an expert by any means.. but something else is at play there i beleive. usually formula will cause constipation because it is highly concentrated. if this is the case, a little cup of water during the day or a little milled flax on baby food will help the formul/constipation connection. this isnt a bad thing - flax has fiber which is required for soft comfortable stools.. too much can cause diarrhea though.. so just a pinch to start.
again, i reiterate, DONT WORRY about how MUCH he eats, just that he eats when hes hungry and stops when hes full - thats much more important than the ounce amount on the bottle. you want him to learn that hunger pains mean eat, and that full feelings means STOP - this will prevent childhood obesity issues! :D
also, in my work with babies, ive noticed that they will EAT AND EAT AND EAT - then they will have a period where they sleep and sleep and sleep! this is perfect and normal for baby growth! they need the fuel, then they need the sleep, hence the reason why some moms can say that it seems like their babies grew up 'overnight' - because they do most of their growing while they are sleeping! '
just relax. as long as you keep in touch with your doctor and there are no NUTRITIONAL deficiencies, your baby will be perfectly healthy no matter how much, or little, he is eating at any given time.
trust your instincts. you will know if something is really wrong.
incidentally, me and my husbands anniversary is july 17, our son weaned himself at 19 months on july 18th - and july 19 your baby was born! interesting huh? LOL