I'm 150 lbs 5'9 and my maintenance falls between 2200-2400 calories...
I would say for his size, that's a bit low and agree on that..
Also, IF is not for everyone and maybe you are fasting longer than you need to as well.. What is your window for eating? Are you filling your macros within these meals correctly and are you getting enough protein? How many grams per C/F/P is your current stats? Percentages don't tell us anything.
Are you getting enough rest or sleep hours?
Also - if you are in a calorie deficit, if your macros are not working with your current ratios and if you're practicing IF then doing HIIT on top of this, I would think you are taxing your body too much too fast..
Idk, just my opinion but it seems something is off maybe diet wise and you're blasting your body with too much rather than focusing on a process that gives slow but steady results..