i feel like it would be odd

i really want to tell my boyfriends dad happy fathers day. he has three sons he has taken care of alone for awhile. he has two that lives with him and one is out of the house but still is helping him with bills and get on his feet while he is providing for two others all by himself. i have a lot of respect for him and i even appreciate everything he has done for my boyfriend and his two brothers but i feel like telling him would be odd because he isnt my dad