In GF II when Connie comes with Merle to ask Michael for money, Michael bawls her out, "Your son was just picked up for shoplifting!!"

Now since GFII takes place in 1958 and the Five Families massacre supposedly took place in 1950 or 1952, Michael Francis Rizzo would have been at the oldest like 8 years old at the time, so it seems to be a stretch he would already be in "trouble with the law."

We also don't know for sure if Connie and Carlo had any other - older kids - when Carlo is begging for his life to Michael, he says, "I swear on the kids," but we never see any other kids belonging to them.

My own opinion, Michael Francis Rizzo Corleone (Connie would change his last name to get rid of any reminder of Carlo, who to her would be nothing but a bad memory) would grow up to be a "bad kid," getting little guidance from his mother, probably being taken under the wing of his godfather Michael during his childhood years, and probably getting involved in crime in his late teens or early 20s, and probably getting sent off to jail in about 1970 for 5-10 years or so (... which would explain why we see nothing of him in GF III, and why he might get involved in the 'family business' in the 1980s after getting out of jail...)