I think RT could be cute, pronounced like Artie! Though it might be difficult for a nickname to stick that’s based on his middle names that completely ignores his first name.
RD, RE, RK and RB also work, but I think RT is the most usable choice.
RT seems the easiest. RZ and RC could work too, and maybe RG? It does seem a convoluted way to get a nickname though. [name_u]Will[/name_u] you not plan to call him [name_u]Leland[/name_u] (which is a great name!) normally?