Constipation in infants starting solid foods is not uncommon, and may not, in fact, be constipation. If your child is not uncomfortable (no obvious abdominal pain, crying, arching his back, attempting to pass stool without success), and is passing soft stool without difficulty every 5 days or so, it's fine.
If, on the other hand, any of the above are occurring, or if the infant is passing hard stool, you should mention this to your doctor. You can try adding non-absorbable sugars to his diet - pear/prune juice, pureed peas and prunes, and offer extra water as well as continuing breast feeding.
The Mayo Clinic has good information on infant constipation.