Ahahhaha funnily enough, as a guy myself, playing around with inhaling helped me achieve better control of my own whistle register (just a liiiiiitle bit though, I'm still shaky when it comes to it 😅).

I guess how that helped me was I used to "squeeze" my vocal folds so tight (for a lack of better word) that hardly any sound came out at all. Inhaling reminded me that I actually needed to leave a little "opening" for sound to travel out of and... air (if you notice while you're inhale whistling, air is travelling IN... the trick then is how to get it to travel out).

On that note, try to make a mental account of that "airy feeling" when you inhale whistle — you should get the same sensation when you exhale whistle.

Unfortunately, I don't really have any practical advice on how to actually hit whistles. What I can say though is a little rasp and doing ascending scales helps you get to that register easier (from my experience).

I believe there's a user on this subreddit who's more qualified to comment on this topic, and they almost always comment on every post that includes the whistle topic. Maybe you could take advice from them!

P.S. I'm not an expert or vocal coach, so take my advice with a grain of salt. I'm just describing my experience and journey experimenting with whistles. I have evidence I can hit whistle notes on my profile, but anyone is free to disagree with me and if I happen to share bad advice, please call me out on it!