There is no advantage in this situation of giving formula, especially by bottle and there may be some important disadvantages. Even at this age a baby may start to prefer the bottle if he seems not to be getting enough from the breast (if, in fact, he will accept a bottle).
On the other hand, if the baby does accept the bottle and if your milk supply has decreased, as it might have, giving the baby a bottle may almost guarantee that the baby will soon refuse the breast. See the information sheet Slow Weight Gain After Early Good Weight Gain for reasons your milk supply may have decreased. It would be preferable in this circumstance to give solids off a spoon rather than to give formula in a bottle. (Frequently, however, this situation can be remedied differently by improving the breastfeeding -- get help!).
If you wish to mix formula with solids, that does not cause the same sort of problem as giving formula or even expressed milk in a bottle. If the baby seems hungry after breastfeeding, feed him solids off a spoon. However, it may be possible with a few simple techniques, to get the baby gaining well, and/or to be satisfied with breastfeeding alone. Check with the clinic.
Questions? Get Dr. Jack Newman's book The Ultimate Breastfeeding Book of Answers.
Jack Newman graduated from the University of Toronto medical school as a pediatrician in 1970. He started the first hospital-based breastfeeding clinic in Canada in 1984 at Toronto's Hospital for Sick Children. He has been a consultant with UNICEF for the Baby Friendly Hospital Initiative in Africa, and has published articles on the subject of breastfeeding in Scientific American and several medical journals. Dr. Newman has practiced as a physician in Canada, New Zealand, and South Africa.
Breastfeeding and Other Foods, 2009©
Written & revised (under other names) by Jack Newman, MD, FRCPC, 1995-2005©
Revised Jack Newman MD, FRCPC, IBCLC & Edith Kernerman, IBCLC, 2008, 2009©