R.J.
A week or week and a half as long as he's not uncomfortable.
Although it freaks once a day'ers out, even many adults only poo once a week. It's a function of metabolism + input = output. ((It conversely freaks once a week'ers out at the idea of going one or more times a day))
Solids are known to cause digestive problems (and sleep problems) in babies whose digestive tracts aren't ready for solids yet. So it this is a "we started solids last week or the week before and now have this new problem" just hang back on the solides for 3-6mo).