Whether he has mental health issues I do not know. I'd assume his behaviour isn't exactly indicative of somebody mentally well, but I'm not an expert.
Whisper it quietly, but: Mental health is a spectrum, and most of it is socially constructed. Anyone who studies psychology or psychiatry very quickly realises that there are a lot of arbitrary boundaries.
We made mental health a medical speciality (psychiatry) in the 19th century back when we didn't really have a germ theory of (physical) disease. Since then we have sequenced the genome of thousands of pathogens, but we still can't say what "causes" depression or psychosis, in any comparable way to how we can say that this or that virus or bacterium causes flu or tuberculosis.
Of course you have people with, say, traumatic brain injury whose behaviour is radically changed by what is obviously a physiological cause, and you have objectively measurable damage to the brain in stroke or dementia patients, but beyond that we don't really know what "depression" or "anxiety" or "autism" are, other than a series of behaviours that we measure with tests of those things.
This is not intended as a rant against mental health professionals, who mostly do a great job, but to some extent they are in the dark like everyone else, especially when it comes to the intersection of "not right in the head" and premeditated violence. We cannot predict whom, among the many hundreds of thousands of people in the UK with psychological profiles not dissimilar to this kid's, will go on to harm anybody, and we probably wouldn't want to live in a society where the authorities had the power to imprison such people on a preventive basis, when they have as yet committed no crime.