S.P.
Wow. First of all, I am just delighted to know
that you get to spend some time with your GREAT grandson,
that you are close enough and well enough and trusted enough.
It seems to me that there may be one or more things
that are not so positive/appropriate with his parents
or whoever his primary caretakers are.
Among the things I might wonder about are --
what kinds of food he eats
whether he is taken care of consistently and lovingly
whether there are stress/fighting/dangerous things going on in his home
et al.
If the purpose of the melatonin is to help him sleep . . .
when was this started? Does he need this every night?
Maybe it's not working or maybe he's grown tolerant of it.
I hope you'll be able to help your great-grandson
in whatever ways you can . . . especially if you're in a position
to be able to improve/enhance his day-to-day routine.