The simplest solution (for now) is a coaster brake + a front hand brake. This setup has been SOP for decades and will serve until he outgrows coaster brakes.
When he's ready you can fit hand brakes with a lever that pulls two cables. These are common in tandem use and work well, but require careful fine tuning and maintenance to maintain good front/rear proportioning.
When he's older there's the option of hydraulic discs. These can be run off a single lever with a Y connector. It's not ideal to run 2 brakes off a single master cylinder because there may be issues with the travel needed. Also, you can't proportion brakes with a common master cylinder, so both will be working equally likely causing premature lockup of the rear wheel. However, that can be compensated by using different sized rotors, so long term, hydraulics may provide the best, most serviceable solution.
OTOH - things change fast, and with your son only 3 years old now, there's an excellent chance that technical advances may make everything simpler by the time he's ready.
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