Op Jonty is the top of my list!
To appease dh, may put Jonathan on his birth certificate (and so he has the option of a longer name if he likes when he is older). But I would plan to introduce him as Jonty. Jack, Jonny (however spelt) don't appeal.
Montgomery/Monty was vetoed by dh in reference to the poster upthread who said they preferred Monty!
I do appreciate that Jonty is not to everyone's taste, judging by this thread 