Hi there - welcome to Purrs, to you and Tonic.
Could be a number of factors here, Scaz, that are contributing to his change of behavior.
Does he have free access to the outside, or does he rely on you to let him in and out? If the latter, he may have found something which has captured is interest, and doesn't want to be "stuck in" if you then have to leave and do something else (like going to bed!)
There may be a "new kid on the block" - as he's neutered it's unlikely he's looking for top totty but he may want to keep an eye on their activities.
Cats get used to sharing territories, but if the hypothetical "newby" hasn't been neutered, or is just a strong personality, Tonic may be keeping a closer watch on his patch.
Or, he may have forged a friendship with another cat. Or he may have found a particular area which is proving "interesting" - he may have found a bird table to watch, or a pond, or one of the other 1001 things cats get fascinated by.
Some cats mature more slowly than others, and it's not uncommon for cats to hit a "teenage" phase, where they test their boundaries with their people, just as human teenagers do.
Do you know what he normally likes to do when he's out, or what his normal patrol patch is? Is he a bit of an adventurer, or has he always stayed very close to home? These things may give you clues as to why his activities are changing.
That said, sometimes cats do change their habits depending on weather and mood. I've found most of my cats have preferred being outdoors in the summer, especially in the evenings, coming indoors for food or a serious sleep (although Moray's just as happy snoozing the day away on a comfy seat in the garden....
) In the winter, they tend to spend chillier wetter nights closer to home.
