My only advice would be to try and be low key. I'm not sure of the specifics with your sitch, but making a big deal is no good. A card, MAYBE a small gift but even so, maybe make it something practical that he needs anyhow. No R talk on the card. Act like a coworker essentially. No matter what, it will likely be awkward.
"Love me when I least deserve it, for that is when I need it the most"
M18 Me39,H42 D16 Bomb 1/10 Moved out 3/10 OW 6/10 H wants to R,OW gone 11/10 H moves back 5/11 H wants to wear rings again 9/11
