Dear R., I'm not a pediatrician, but I can tell you that my daughter's pediatrician (also a mother of 5) would have had a STROKE if I suggested feeding my infant every ninety minutes. Childhood obesity is a serious problem, and establishing a pattern by which he believes he "needs" to eat that often could be setting him up for a lifetime of health / food-related issues.
In fact the two children in my daughter's playgroup who were nursed "on demand" both grew into OBESE children -- one of them tragically so. (we started this group when they were newborns at Georgetown hospital, so I watched these kids grow up).
Now I do understand that good, well-intentioned people often differ on this topic, and my "two-cents" isn't meant to cause offense...
As I'm sure you've guessed, I'm a member of the camp which believes that feeding infants "on demand" is less than ideal for both child and mother. Sometimes he may just need to suck on something, but may not require a meal. Other times he may be looking for comfort (and when one feeds on demand, it is natural for a baby to confuse
"comfort" with food).
One of the commenters stated that it is impossible to overfeed a baby who is exclusively breast-fed. I've heard this before, and still fail to understand logic. Generally speaking, assuming you are healthy, etc (and I've discussed this with several peditricians who also happen to be mothers), your body will produce as much milk as you "need" to feed the baby. When you nurse more, you produce more milk; when you nurse less, you produce less milk...if this is true, than our body chemistry does not "automatically" prevent us from overfeeding.
Also, you are part of this equation. You need your sleep! Sleep deprivation can tamper with your immune system, it can trigger mental health problems, and it is cited as one of the leading causes of car accidents, and other types of accidents. Of course every new mother is sleep deprived to some extent, but you don't have to exacerbate this if you don't want to.
I'm sorry that I don't have any good advice about how to rearrange his feeding schedule (if that's what you're inclined to do), but please do get some professional feedback. And if you end up talking with the La Leche League or their followers, please also make a point of getting some input from other professionals who might be less militant about the subject.
Best luck -- it's obvious you want the best for you and your baby!