Well - you asked a specific question, so here's a specific answer:
Your baby may have slightly less white matter in his brain, according to this latest piece of research listed by UNICEF:
"Breastfeeding and early white matter development in the brain
This study compared the amount of white matter, which facilitates the rapid and synchronised brain messaging required for higher-order cognitive functions, in the brains of 133 healthy children from 10 months to 4 years of age using magnetic resonance imaging (MRI) scanning, and compared the results of infants who were:
exclusively breastfed for three months
exclusively formula-fed
mixed fed with breastmilk and formula.
In addition they examined the relationship between breastfeeding duration and white matter microstructure.
Results of this study describe some of the earliest changes in human white matter development, including an association between early exclusive breastfeeding and improved developmental growth in the late maturing white matter regions of the brain. This area of the brain is associated with higher-order cognition such as planning, social-emotional functioning and language.
Extended breastfeeding was positively associated with improved white matter structure and cognitive performance, including language performance, visual reception and motor control.
Whilst the mechanism remains unclear, breastmilk is rich in long chain fatty acids found in breastmilk (docosahexaenoic (DHA) and arachidonic (AA) acids) which promote neural growth and white matter development. Formula and weaning foods only contain the precursors to these.
In conclusion, the authors state that this study provides further support for the hypothesis that breastmilk promotes healthy neural growth and white matter development.
Deoni SCL, Dean DC, Piryatinsky I et al (2013) Breastfeeding and early white matter development: A cross-sectional study. Neurolmage. 82. 77-86."