Do you know his Apple ID password so you could do find my iPhone without family sharing? If not can you not ask for it in a laid back way. Like a purchase you made on your child's iPad isn't working, you'll just quickly log in on his Apple ID and try to retrieve your purchase or some excuse?
DO NOT let him know your on to him, and give him time to hide his tracks. Very little men confess of cheating. Even with evidence they gaslight and try to worm their way out of it.
How about a car tracker? I've not used this method, so I'm not clued up, but I've seen people on here use them effectively.
Have you tried just simply going through his phone to find your answers? If he has an iPhone you can go in to settings > battery and then view how long he's spent on what app, and what time and day. Have a look on Fridays between the times he's saying he's busy?
id just go through his phone as soon as I got the opportunity. It's really not invading someone's privacy when they have a history of cheating and have no problem making their wife look like a mug.
how about calling the uni and pretending to be someone else and asking to speak to "mr so&so" whatever his name is? And say it's a student or parent and it's urgent or whoever you can think off. See if the secretary says he's in a meeting at the moment? Or you could say does he have a meeting at the moment or is he available to speak, as if he's in a meeting I can wait? 😂
Sorry I probably sound crazy but I've been in your shoes and knew something wasn't right but DH covered his tracks well, so I had to go the lengths to find the evidence. And it drives you crazy 🤣