yeah loads of teenagers when I was one liberally used words like the one he used/homophobic ones pretty liberally as insults, teenagers aren’t exactly known for being worldly and knowing the real depth of meaning and connotations behind things (I mean up until fairly recently ladyboy jokes weren’t seen as being particularly offensive), as long as he learns its ok
I think he knew exactly what the word meant as he impersonates Harry Kane with an exaggerated slow speech. That being said, this isn't about throwing someone under the bus, it's about taking the opportunity to promote a greater understanding of equality and diversity. He's going to be in the media eye much more now and with that comes a responsibility to act as a positive role model. I'm sure the club will already have spoken to him in depth about why this video is an issue, they'll give him advice about how to avoid these pitfalls and give him opportunities in the future to meet with fans with related disabilities. With all those things, we move on. I'd only take exception if this mistake is repeated in the future, in which case it would be indefensible.
As reddragon has eluded to... Offence isn't determined by the recipient or the offender. It's determined by anyone who hears the insult, and with the context of that word as a derogatory term for a particular group in society, I can completely understand why it is so offensive to a great number of people (not just those with connections to people with learning differences).
I have to say though, I hope the person who distributed the video has consequences too. Plus, I'd be interested what Fulham's academy and club staff have done to raise awareness of social media related issues and the equality based issues this highlights. There is a culture in some football circles that doesn't exactly challenge this sort of thing very well.