Has he listened to other children or adults playing the violin? Has he been to children's concerts? Do you have some violin CDs so he 'knows' the sound?
If this is the case, and he wants the violin (and doesn't just say it once, but often over a certain period of time), let him play the violin and not the recorder or the guitar.
If not, I'd really try to find another (even a bit older) child who could play to him, or is there a possibility to let him listen at school concerts or something like that? Our local Music Trust sometimes does Instrumental Taster Days (don't know the exact name of it), maybe there's something similar in your area? It would be really good if he could hear children (or if not possible, adults) play, ideally not just the violin but other instruments, too, so he gets a clearer idea.
Some children know very clearly which instrument they want. I knew someone who would have refused to learn anything but the violin (her parents wanted her to start with the recorder - she said she was going to play the violin or nothing at all, and her parents gave in...). As an adult she became a professional orchestra musician.
At that age, he'd need very short lessons though, no longer than 15-20mins max. If there's a waiting list, maybe you can put him on it and by the time he gets a place he'll be nearly six and the perfect age for starting.
Be prepared though that the violin (as any instrument) requires daily practice, ideally, whilst the child is young at least, with a parent who sits in the lessons and practices with the child - if done properly and if you/your ds want(s) to see some progress. Daily practice can be a couple of minutes to start with then graduallly building up, and of course later on the parent doesn't need to sit all the time with the child. If he doesn't practice (apart from the occasional missed day of course, and at that age I wouldn't be overly strict) it would really be a waste of money.
At 5 he is definitely able to gradually learn to read music while learning an instrument, if he's otherwise academically brilliant, so that shouldn't be a problem at all.