TLDR: the reasonable and responsible thing to do is to see if what he means really is indiscriminate hatred towards all Muslims, or whether what he genuinely is concerned about are the numerous negative effects of a growing Muslim (and increasingly Islamist) population in Britain. Responsible and charitable interpersonal behavior requires us to take each other's concerns seriously and see if and in how far we agree with them, and point out in a manner that has a chance of being heard where we disagree. Appeal to shared values, acknowledge pertinent facts and valid concerns, and be gentle, yet decided in objection to blatant falsehood.
Take his concerns seriously. Talk to him about what specifically he believes about Muslims, and appeal to valuing facts - and be willing to listen to the facts (or claims) he considers pertinent. You won't be able to change someone's mind by condescending to them, or by calling them bigoted, or by showing a disregard for their opinion that brushes away everything that seems relevant to them without genuine consideration. You will have to engage the ideas in a rational and empathetic manner, and concede cases where you see that you may not agree with him, but where he does have reasonable concerns. None of us has their thoughts sorted out enough or is able to articulate them clearly enough in all cases that we could have conversations about things we disagree on entirely without friction, so do your best to minimize the friction that comes out of sloppy thinking or speaking by being charitable in listening and interpreting his words; ask for clarification where you're not sure what he means.
Does he think all Muslims beat their wives and have sex slaves? Introduce him to some Muslim friends who clearly don't, and acknowledge that there are worrying trends in Britain in this regard. Developments like the child sex slave rings in Rotherham and other places (just one example of many grievances that could be amplified to an extreme position like "all Muslims do X" in tabloids) that were ignored in large part out of a fear of being labeled racist and bigoted show that condescending to someone in that manner has disastrous effects, and from an interpersonal point of view are necessary to acknowledge as valid grievances. Don't concede extreme positions like "all Muslims rape children", because they clearly don't, but don't swing to the other extreme of "nothing to see here", that can only be received by him as being delusional and a judgement of his moral character.
Does he think all Muslims hate gays? Introduce him to some gay Muslim friends, and together talk with them about why mainstream Islam condemns them and teaches they deserve severe punishments, what reactions they experience from Muslims, where they see opportunities, what values their allegiance lies with, and how they reconcile themselves with Islam.
Does he think all Muslims are Islamists? Talk about what exactly he means by that - support for stoning adulterers, jailing/executing blasphemers/satirists, flogging people who have extramarital sex, implementing Islamic marital law, etc.? There are numbers on what proportion of Muslims say they subscribe to those notions, how many support sharia in general, etc. Those numbers are worrying, yet they also show that many Muslims do no support those specific demands. Acknowledge the concern and explain why you think it is less worrying than he thinks. If he disagrees with your explanation of that, recognize that it's a matter of anticipating future behavior of large groups of people, and that it's by no means clear whether a more optimistic or more pessimistic evaluation will turn out to be accurate, so disagreement on that point, to a degree, is perfectly fine.
You may not be able to adjust his views to exactly yours, and you shouldn't aim to - he will see that as domineering, condescending, moralizing, and obstinate. He will see you as stubborn and as closing your ears to what he considers valid concerns. A friend that can't rely on you being willing to listen to his concerns and acknowledging that he may have a different, but also valid, view on things than you won't be your friend for long.
