I agree with you, and want to add some more about this. Usually the 90 day things are from companies that also want you to sell your house and relocate to where they are. Often you do a fantastic job and are let go at the end of the 90 days because 90% of the time this scheme is how disreputable companies hire contractors for 90 day stints while paying low rates with no benefits.
Now sure, this guy is the rare 10% that is completely honest and pure, I will assume that and he has no ill motive at all. But the other 90% that he has no control over is the real issue with this. Most people with talent that have been around the block a couple times have gotten burned by this scam and avoid these scenarios like the plague. If you are a reputed expert the way you handle this is either handle it as a straight consulting job at your standard rate (which should be something like $180-$250/hr if you are any good at what you do or $120 if you are a newbie) for the 90 days, or you put a big fat penalty in the contract that if they say no at the end they have to pay severance to cover the full cost of both of your relocations, the one out there, and the one back, including losses from the required hasty real estate transactions. Will they agree to these? Usually not, and that's the point. You really only want to do business with people who are fair dealing, not people looking to cheat contractors through false promises.
Another scam is the two day long interview where you work on some serious problem the company is having and it happens that you are known as an expert in that field, your name coming up on some searches. For me it was Windows graphics drivers. Back in the 1990s I would get tons of job interview requests and then would be treated to two days of intense querying to "prove" that I knew how to solve their problems in detail during which they either took a lot of notes or recorded the interview. Lots of other people get this one too.
Another related scam is the programming problem you are supposed to solve which should take 30 minutes, but as an expert it takes a full week. Instead of being a puzzle it is something obviously related to their business.
Just say no to all these! I know the bright eyed and bushy tailed kids won't listen, but this old geezer has to warn them nonetheless.