Feed him lots. He is be okay for a bi but not long at all. Atleast you have good filter on your 55g, I see it's a FX5. Having good filtration will make him grow even more. I bought my 1.5 red arrow last december at 6", now helot bigger, out growing the 135g he is in. So be ready for an upgrade.
Have you named your arrow, I call mine steve.