IMHO you should be riding a 52cm.
Actually, the 54 is probably much likely to be a good fit for him compared to a 52, in my opinion and experience..... depending on how much reach and bar drop he wants especially. It would be very, very difficult for him to get his bars where he wants them with the shorter head tube on the 52 (assuming comparing a 52 and 54 of the same brand). To make any meaningful advice to his sizing, you'd need to know how the "54" compares to the "52" in the two most important dimensions - TT (or ETT) and HT lengths. Since "52" and "54" generally refer to the seat tube length, it's really meaningless to draw a conclusion based on it alone.
The current fad to undersize frames for the tall seat tube style usually results in difficulty getting the handlebars up where many (I dare say most) recreational and enthusiast riders would be most comfortable and therefore most efficient for their riding.
Of course I'm biased based on my own experience. I've always ridden 54's with ~54 - 55 cm top tubes and they fit me perfectly. 52's (again, given a given brand so that a 52 is scaled down from a 54 in the two most important dimensions - ETT and HT) are too small. I'm 5-8 barely, fwiw.
"Frame size" as it usually is based on seat tube measurement is irrelevant because the only thing it can even begin to estimate is how the person will standover the top tube, in other words, crotch clearance. Crotch clearance on a road bike only needs to be "barely enough" and what the OP describes is perfectly adequate if it is what's needed to get the saddle-bar drop he needs.