Mrs. J does have him work on an exercise for a week or two that will make learning the song easier... but pretty much as soon as Delta is taught the first line or so, he starts trying to play more and more of the song.
Also - although not every practice is done totally joyfully, they mostly are! Delta did get upset about doing the twinkles the other day - I don't remember exactly what he was upset about - but he was playing them all mad, so his technique wasn't good - so I took the violin away and told him that he couldn't play it mad, and that I would do Echo's practice with her. If he was calmed down after her practice, he could finish his practice - but he wasn't going to play it mad. It worked - he played the other songs (after Echo was done) happily.
He is also sometimes getting out the violin and playing at other times - when his dad is getting ready for work (his dad works nights), or increasingly when he is supposed to get ready for bed.... I am happy about it, although not when bedtime is already very late!
Also - I have started playing with him most days during practice (except during tonalizations, and the new exercises) - he really enjoys it, and has really made him enjoy doing review songs. Echo is a bit upset that I can't really play with her as much - but I have to help her a lot more.