This will sound controversial to some, my interpretation of what remaining English means, has nothing to do with race, colour or ethnicity and I don't believe it is part of the context of Brexit either and maybe should not be a subject for this thread. It has more to do with upholding British/English culture and law as we, as a nation, have developed it over a number of centuries. Now, both culture and law are both subject to interpretation as we all know, which is why I don't think it's a discussion for a Brexit thread.
Whilst change has been undoubtably happening since the dawn of time, there are certain aspects of culture and law that should not be subject to such radical change that it creates a shift in the overall well being of the general population, creates unease, distrust of institutions, of governments abilities, questions the mental health of our leaders, of one's self and one's own position in the world. Change for the better should create more happiness for everyone, that hasn't been happening for a few years now and is surely a contributor to the source of your security guards inability to assert an opinion?
I can understand why your security guard is worried about the losing of the identity of the UK and is unable to articulate why, if he has been following UK events over the last few years, he must be very confused about what is happening in the UK and the wider world. Did you ask if he read the MSM by any chance? That may be his source of confusion, if he reads/listens to MSM then he will be missing a large part of UK/World events that are underreported/not reported at all and what is reported is subject to media manipulation.
If he is opened to a wider view, then his enlightenment may bring some focus to his feelings and he may be able to articulate his views better. Although articulating some of those views is becoming increasingly controversial in the current climate of re-defining free speech.
So the feeling the security guard has is one of unease? And his default position is Brexit? Then my advice would be to examine his source of confusion, to seek new sources of information, ultimately to go through the stages of disbelief at what is happening, then acceptance, then understanding, so that finally he will be able to make a better judgment on which to act.
This would be a dangerous suggestion for a Remainer to make though, for the security guard would surely remain a Brexiteer.