People often are confused by the difference between a KEYBOARD and a DIGITAL PIANO. Although the keys look exactly the same on both, the methods of PLAYING and LEARNING are TOTALLY DIFFERENT.
To play an accompaniment on a Piano, the student has to WORK, and make full use of the left hand. On a keyboard, the accompaniment can be achieved automatically by ONE FINGER of the left hand (though other ways to playing chords and backings can be developed as the student progresses.)
So, for a 6 yr a Keyboard is definitely going to lead to more enjoyment, satisfaction and progress.
[I have replied to this topic many times, so if you search my name plus the relevant music keywords you should find plenty of advice.]
I had never heard of 'Dogs and Birds' but have had a just had a look at it. I am sorry to have to tell you I consider it totally expensive RUBBISH.
I was involved in music for over forty years, and taught children on a various instruments for twenty years.
I assume he is learning to READ and WRITE at school, and is using Phonics. If he (and you) can master that there is no reason why normal, conventional printed music cannot be learned, without recourse to flashing lights and unnecessary gimmicks.
(I will give more advice at another time.)