so i think joji's love language would be two things: attention to detail and physical touch
he'd remember really small things that you'd talk about and ask you about it, he'd order your favourite food to your place of work out of the blue, buy you that thing you said you liked in the window display once, develop an interest in something you like on the dl because he knows how much it means to you, wear something/do his hair in a way you said you liked etc, see how your body reacts in bed, make a mental note of it and do it again bc he just loves to please you
i also feel like he wouldn't be too good with words (strange for a song writer i know) but maybe he'd feel awkward about expressing his feelings out loud so he'd do that with physical touch - him being the little spoon, giving you really long warm and tight hugs, caressing and squeezing your butt in a non-sexual way, giving you little kisses all over your face to express his adoration, cuddling you when you're working on something, playing with your hair while he's on his phone, just laying in bed and looking at you and smiling stupidly, basically just try and be as close to you as possible when you're together because some feelings just can't be put into words