they are good for cause and effect, but have seen them used as a pre-cursor to speech/ communication too. i knew a little boy who had two which he used for yes and no for a while. (he had some limited arm movement but not any real head control). depends what you put on them i guess!
that's the next step though, you are right, giving choices and 'insisting' on some form of communicated choice. it will be easy enough if he's eye pointing, but might lead to more - definitely need to move onto 'do you want x or y', dgs. 
dd2 had a communication book for a while which hung on her chair and went to nursery etc. a bit about her and favourite things etc, and a common choices bit at the back with pictures, so that if she went into meltdown they could at least attempt to work out what she wanted using the pics in the book 
with makaton, it was singing that made dd2 interested
she could join in with songs and sign. trying to do old macdonald in the car looking in the rear view mirror to find out what animal was next was a bit hairy, but it was a definite stimulus for her to be able to join in and 'lead' instead of being a passive recipient... that started at the communication group. we had a bag for each song, with different items in, so for old macdonald there were laminated pictures of different animals, as well as plastic toy animals. depending on developmental stage, the kids whose 'turn' it was to choose either got a free hand in the bag, or two pics etc. then the choice was reinforced with both sign and speech 'oo, a pig!' and the sign. so to start with dd2 would be given two pictures and would have to gesture towards one of them. (children who didn't respond were given time, and then one of the oicture would be touched onto their hand and the same process - 'oo, a cow!' and the sign etc.)
you've probably been through all this.
sorry. just really wanted to say it was singing, really, that got dd2 started with makaton. but she did ignore all signs for months before she tried it.