DS specifically asked to learn electric guitar, because he "wants to learn to play the Beatles' music", which mitigates the acoustic quality issue. When I checked sizing guides, he is on the low end of the recommended height range for 1/4 guitars, which are typically sized for 4-6 year olds.
I agree with everyone else, I would wait and see. But if you are going to start a preschooler on guitar I would not start one on an electric guitar. I took guitar when I was a high school and it takes a while of practice to get get the calluses that make it comfortable to play. I wouldn't even start him on a steel string but a nylon string guitar. We had until recently a decent child's nylon string guitar, for elementary kids that my son took lessons on for a while when he was 8. It takes a lot of practice and sore fingers to get used to holding the frets down, particularly on the steal strings on a eclectic guitar. I would suggest this even for an older child. What you learn on a acoustic guitar does translate to electric.
Most Beatles music can be played on an acoustic guitar.
