I have the same restriction so I am familiar with the options. Yes, he can train and get his PPL no problem. He simply will not be able to fly at night as PIC due to the restriction on his medical. His options are:
1) find a good AME like Dr Chen that has several of the approved vision tests and see if he can pass one of them, thus removing the restriction off his medical. Note that he will have to do this every 5 or 2 years every time he renews his medical.
2) Let his medical elapse and go with Basic Med. It's up to your brother in law to self certify that his color blindness is not a hinderance to flying safely at night.
3) Obtain a SODA from the FAA. You meet with an FAA guy, you look over a sectional and he makes sure you can see and point out the different colors on the sectional. Then you go to an airport during the day and take the light gun signal test. If you pass, you have a lifetime exemption to the color vision test on future medicals. If you fail, you have the option of retaking the light gun test at night. If you pass, your restriction reads something to the effect of "Not valid for daytime flying by color signals". You are allowed to fly at night, the only restrictionis you cannot fly by color signals in the day. Example: Your loose coms during the day at a class D airport, the tower realizes you are NORDO and gives you light gun signals. A normal pilot would follow the signals and land. A person with this restriction would not be legal to land and have to go to a untowered field (if safe to do so of course.) If you fail the daytime and nighttime signal gun test, you have a permanent restriction placed on you and you will never be able to fly as PIC at night. As you can see, with such high stakes, going for the waver is the last straw, and only take it if you are sure you can pass.
