if he's struggling to sing a note back to you, it sounds like he might benefit from some exercises to train his ears and try and get a connection between what he hears and what he sings.

I'm not a music teacher so have no idea what exercises there may be designed to help with this, but off the top of my head it sounds like what you're already doing (playing a note at him and getting him to sing it back) is a good place to start. Also, playing two notes and getting him to say which is higher or lower. Plus interval recognition, singing scales and arpeggios, getting familiar with the concept of seconds, thirds, fourths fifths etc, and then playing a note and getting him to sing a third above or a fifth above or whatever.

The thing he needs at this stage is feedback - was what he sang right or not? and if not was it too high or too low...? Not sure whether an electronic tuner type device would help with this so he could practice on his own, but they might be too sensitive and therefore unhelpful. Your ears are probably the best measure of whether he's getting it right or not - so doing a series of this type of exercise as warmup at the start of every practice might be helpful.