I consider 6 to be an absolute minimum for any shoal or school, and I personally prefer to not keep fish in groups of under 10 if they're schooling. Also, it's probably worth keeping in mind that H. rodwayi reportedly live for only 3-5 years, so if he's already an old fish…
Have you gotten a positive ID on your fish? Does he have white-tipped dorsal and anal fins? And the black spot on the caudal, with a bit of red top and bottom of it?