There's definitely an argument for it being racist mind. Quite simply, if he was a white lad it wouldn't be sung. We all know that some people are singing about his cock because he's black, and there's a stereotype that black people have big dicks. A few years ago it was sung that nyron had a massive cock that he swung around his head. It wasn't sung about Jonny Evans.
I remember reading something about this when man united fans were asked to stop singing about Lukaku's cock. If I remember right, a main part of the argument is that if people are accepting of a "positive" stereotype about a group of people (black lads have huge cocks, they're athletic etc) then they likely also believe the negative stereotypes (they're less intelligent, they're violent etc). It's not something a player will necessarily be comfortable with hearing
It's pretty likely that a player would rather people sung about a quality he has as an individual, rather than just had a laugh about a racial stereotype regardless of whether we think that it's harmless, or meant to be supportive/ positive. Certainly now that the lad has publicly asked for it not to be sung, it shouldn't be. As others have said there are plenty of alternative lyrics that fit perfectly well. I'd also add that any song which could be perceived as racist should probably be avoided when it's primarily coming from the same place as (a minority of) people who boo taking the knee, it's not a good look