Cats scavenge all types of foods and hardly ever come to any harm, so I can't think that a bit of a human's biscuit now and then is likely to be of any great concern.
I've got three cats myself and I've read loads of web pages over the years about foods that are dangerous for cats. Biscuits have never featured in those lists! (The sugar content might not be good for his teeth but it's probably no more than he'd get from eating proper cat treats, like Dreamies, which contain sugar too).
Even foods that can be harmful to cats, such as onions, aren't problematic in small quantities. (i.e. while you shouldn't feed a cat a regular diet from cheap, out of date, cans of beef and onion you might buy from a discount store, letting him occasionally lick the last drops of onion gravy off your dinner plate won't harm him at all).
I find it odd that a cat should like malted milk biscuits. (Perhaps it's the sugar in it that he likes?). However I wouldn't be at all concerned about a cat being fed a bit of biscuit as a treat.
Relax!