It's kind of a personal matter. Neutral wall colors give the best options. I like a white with yellow undertones to bring some "sunshine" into a room, but other people get anxious if they get too far from grays. Anyway, with a neutral house-wide wall color, then he can decorate each room to his taste -- only those that can be viewed at the same time really need to harmonize. Maybe a good place to start is with a couch -- could go with gray, denim blue, some olive-ish green, whatever he finds, finds comfortable, and can afford. Then he can find a rug with some pattern that harmonizes with the couch. Then he has a few landmarks for filling in the other parts...

We have no context . He needs to postpics of the space . Give us some idea of what he NEEDS in each space .. He can aslo start his own dilemma and leave Mom out of it. I had 3 sons and in no way would ask strangers how to help them with decor. We need so much info from him so if he does not want his own dilemma then you will need to amnswer questions, show us the spaces and provide a to scale floor plan of the home . Color palette comes much later in the process . He needs to go shopping to find furniture HE likes that is the start . We need to know what he chose . I add things like rugs and drapes after furniture and it just works better . Maybe at least get him to come here under your name to answer some questions. Honestly does HE know what tranitional decor is ???

steer away from gray.

a 24 year old man should not get started collecting drab, boring furniture, no matter the style. Look at his closet, what color clothes does he buy?

Chambray? french blue shirts? Leather? Set the tone for his own style reflected in his home. Is he a sports fan? travel? put together a vision board of interests and colors, don't just go out a buy a furniture display, FYI they are generally too large for a normal home application anyway.