The letters intimidate him because he doesn't recognise them or their meaning. Only introduce them when he demonstrates that he is ready.
Ask him what his initials are. When he tells you, ask him if he would recognise he was being addressed if you used only his initials. When he (hopefully) says yes, then ask him if distance might have an 'initial' he could suggest so that when you used that letter, he would know you were referring to distance. Hopefully he will suggest 'd'. And so on.
You could even introduce the lesson by having every student only address other students using their initials... good fun, but it breaks down the resistance to the dreaded 'letters' :)
Then only introduce variables which have immediately obvious 'initials', such as p for price or pizza, h for height, l for length, a for adults, h for hours, etc, depending on the context.
Only after he has begun to associate letters with practical measurements and quantities should you mention that 'algebra is just the same... it just uses abbreviations to simplify things, just like his initials.
Any talk of x and y and z will re-introduce the fear of symbols. It's better to wait until he gets frustrated with 'all these words' and the idea of an abbreviation is his idea...
I often tell students that they have been using algebra 'for ages', only they didn't realise it. Then I tell them a word story like this:
Jimmy goes to the bank and gets out enough money for three pizzas. On his way to the pizza shop, he finds 8 dollars on the footpath (sidewalk). When he gets to the shop he has 41 dollars. How much was each pizza going to cost him?
Almost all students who can add and subtract will get it correct very quickly.
Then say "I'm going to write the algebra abbreviation for that story, and write 3p + 8 = 41 on the board.
Say "if he hadn't found the 8 dollars, would he have more or less?' They all will say 'less'.
3p = 41 - 8
NB! don't introduce any rules about sides and signs and what you do to one side, etc, same for words like solve and expression and equation. These should only come after the process is understood using things they already know... or you'll turn him off all over again.
and ask how much money did he get from the bank? They will say '33'
3p = 33
The say 'So, how much was each pizza going to cost? They will say '11 dollars'. Trust them to refer to what they already do at the shops, not what you have to tell them about dividing, or whatever. Trust that their experience already contains algebraic processes, only they didn't call it that... but they were perfectly good at getting it right. Hint! money and food questions are very familiar to almost every student everywhere.
Then say 'That's all algebra is: an abbreviation for words, using a consistent set of rules.' They will get it... and then wait for them to ask about or discover or intuit or teach each other the rules. Teaching is as much about asking and listening and enabling as it is about telling.
Then make up similar stories that you read out and they write the abbreviations for. Include things like 'on the way he lost 12 dollars', etc.
I have done this for 25+ years and it it has 'worked' almost every time with almost every student, so long as they can add, subtract, multiply and divide competently. If not, then address that first.
Only introduce abstract concepts and 'letters' when students are more than comfortable... and you have exhausted most of the other letters and they are ready for some meat in the sandwich.
Hope this helps