@ri99 - I'm not T1, so I am not posting from a diabetes perspective.
If you say that you recently noticed he was looking a bit thinner, and when weighed he had lost 3kg, the do you think the weight loss has been really slow, but somehow you just noticed it?
I think everyone's weight varies a bit, just day to day. We have high days and holiday, or days when we eat a lot (think Sunday dinner, versus a light evening meal on another day), and we have "not too hungry" days.
If I have a couple of really hungry days, as I did last week, I can gain a 1kg, but as soon as I get over the hunger and normalise my eating, it just disappears again.
To be honest, if all other factors are good - he feels well, bloods are good and he hasn't been exercising any differently, or under stress, it makes sense to mention it to the medics; just in case. They're always interested in weight loss.
Until the appointment comes around, maybe he could adopt a step-on/step-off routine every morning, to just track his daily fluctuations to ensure he hadn't weighed on a "light day".
Obviously, that doesn't explain why his waistband could be loosening, but worthwhile all the same?
I'm a very slight person and I do the on/off routine every morning, then once a week I do a proper weighing, which measures more metrics than simply weight. That probably sounds bonkers, but I tend to lose weight if I'm not careful, and it takes a while to gain it back.
But, to reiterate, unexplained weight loss shouldn't be ignored.
Good luck with it all.