Al,
My suggestion is the same as Roberto posted. Driving will NOT be faster than the high speed trains, which travel at up to 300 km/h. The travel time from Padova to Milano Centrale via FrecciaRossa is 1H:57M, plus the time & cost of a Taxi to Linate, and the 07:18 departure would get him to Milan at 09:15. This is easily the best option, IMO.
There are a few "caveats" to be aware of with rental cars. It's important to note that for driving in Italy, each driver listed on the rental form must have the compulsory International Driver's Permit, which is used in conjunction with your home D.L. These are valid for one year, and easily obtained at any CAA/AAA office (two Passport-sized photos required, which may be provided by the issuing office). Failure to produce an IDP if requested can result in fines on the spot!
You may also want to have a look at some of the other posts here concerning the dreaded Zona Traffico Limitato areas that are becoming increasingly prevalent in many Italian towns & cities. EACH PASS through one of the automated Cameras will result in a €100+ ticket, which you won't know about until several months after you return home! This website provides more information - http://www.slowtrav.com/italy/driving/traffic_cameras_speeding.htm . There's also the issue of parking tickets, high fuel costs, tolls and automated speed cameras including the devious Traffic Tutor system which measures not only instantaneous speed but also average between two points. Violate either or both parameter and expensive tickets will follow.
There's also the issue of traffic delays, which there's no way to predict regardless of what websites such as Rome2Rio or others might say. I saw a good example of that in a travel magazine I was reading this morning. A family going from London to Gatwick to connect with a flight were delayed in traffic. They weren't able to notify the airline and ended up missing the flight. As a result they had to buy new tickets at a cost of $11,000+ (they eventually got a partial reimbursement after appeal). The person writing the story suggested that they might want to use the Gatwick Express (train) next time, as it's not subject to delays of that type. While your brother's situation won't be that severe, missing a flight would still be a real nuisance!
Finally (and perhaps most importantly), taking the Freccia trains will be cheaper if he buys advance tickets online and gets a Super Economy ticket. Using an arbitrary date in May, they have tickets listed from Padova to Milano Centrale at €14.90 PP. I seriously doubt that a rental car will be cheaper than that!