From the few games and appointments invites I have seen for my son, our local Premier League academy doesn't seem to use teams of 3 for U11, but does by U12, and it seems to be a good environment and good challenge for a new official.
As JamesL says, for the latter stages of some cup competitions (including schools competitions in our area) the organisers may arrange neutral teams of 3 even at U11/U12.
Another thought: As U11 / U12 is the typical age for introduction of offsides, football in many geographic areas will require assistants at that age (typically one reluctant parent from each team acting as a 'club assistant'!).
Therefore, local teams may well welcome the offer of a qualified official volunteering to assist in their U11 / U12 games, and would also be keen to support a young official's development. Whilst it wouldn't (normally) be paid, and wouldn't be the same as operating in a full neutral team of 3, it might still be worthwhile experience both of assisting, and perhaps even more of observing how other more experienced referees work. It might also help to get your son's name / face known locally by officials and clubs as a keen and reliable official.