P.M.
As a lactation advocate - I'll give you the other side argument. **Replies to uneducated Moms responding to me below***
1) Babies got thru multiple phases where Moms assume they are self weaning, when in fact they are so busy learning to do active motions and mind is moving so fast assimilating new information - that it seems they are uninterested with breastfeeding. That couldn't be farther from the truth. Human babies do not self wean until AFTER 2 years old. The bottle is instant gratification without any of the effort. Breastfeeding takes emotional time, physical time (suckling takes more jaw strength and suction) and time spent away from being an energizer bunny.
2) Momma's Milk will change a bit during pregnancy, and may dwindle - but you will not dry up. As for the pre-term labor and breastfeeding - contact a IBCLC, not your OB or a regular CLC. Even a CPM (Certified Professional Midwife) would know more about it than your OB. Yes, I'm being very serious.
3) I'd advise you continue breastfeeding during those times of calmness (like the early morning, before and after naptime and bedtime) and allow for supplementation with the bottle or pureed foods ONLY when nursing isn't working or satisfying to your son. Unless otherwise counseled by a IBCLC or CPM, I'd continue breastfeeding as long as possible. Tandem nursing also helps with siblings bonding too... kellymom.com is a great site for many things breastfeeding and tandem nursing.
4) Just because your 'formula fed' baby is healthy, happy, etc - that is known as the exception. ALL STUDIES DONE show breastfed babies are healthier overall, better adjusted and socially adept. I'm a Nursing student with the intention of going all the way to ARNP, and the addition of CPM. I don't say things lightly, nor without knowledge. Most formula fed babies get food allergies, irritable bowels, horrible gas, diarrhea and GERD. Formula is cow protein which is larger and much harder to digest for babies. Cow protein also introduces foreign bacteria into an immature and sterile gastric system. Breastfeed is neither foreign material nor hard to synthesis and keeps the gut of the baby in good shape.
5) OBs do not practice normal births. They are specialty SURGEONS. They see disease and problems where there are none. She requires an OB due to her previous fertility treatments and possible pre-term labor. Most OBs are not trained, nor research breastfeeding in any capacity. That is why a CPM or IBCLC who is CROSS TRAINED with actual graduate level medical research and knowledge is the best person to ask.
6) Mammals are breastfeeding creatures. Humans are one of the longest breastfed developmentally speaking of all the mammals. Only a few breastfeed longer. I understand Moms WANT to assume nursing strikes are actual weaning... but that is on you. I am giving factual information while others are giving completely personal opinions.