Be careful with that; this is something to think about. If he's thinking about only doing 4 years - there's really no point in branch detailing, because by that time you get to that second branch, you're out of the Army (or so you desire). Furthermore, unless he's interested in a combat arms branch (for the most part), that could potentially be a bad move; since he'd spend his entire Army time doing stuff he wouldn't like. This could lead to a miserable quality of life, etc.
I wouldn't necessarily say one should go out of their way to branch detail for the sake of it, just because you're interested in career Army. Sure, getting different experiences is great; broadening your skillset, experiences and all. And sure you could be SC, branch detailed IN; and that probably helps when you're operating at the tactical level. But then look when you're at the field grade level or higher; having that experience as a IN PL isn't quite as relevant anymore. That's just one piece of the puzzle; now you're looking at the operation and stragetic level. You're not concerned with just that IN unit. Now you're looking at Unified Land Operations; potentially integrating (Combined Arms) combat arms units: AR, AV, FA, ADA, etc. But don't forget you need your sustainers too: AG, SC, OD, QM, etc. Now lets take a step further and talk about Multi-Domain Operations. Now we're talking about interfacing with USAF, USN, USMC, cyberspace ... and soon to be Space Force.
Don't fret over this too much. As kinnem said, just take it one step at a time; he's got time.
Just some food for thought.