Why don't you try just asking him directly to hear what he thinks?
assitant
I can see why 69 could be a funny number. When written, the

number has a clear pattern and rhythm to it, and it sounds somewhat repetitive:

It's possible to generate a list of 69 and keep repeating items from that

list, for example by adding a "9" to the end of it.  And 69 is also

pleasingly round, and it was also a favourite number for Albert Einstein.
assistant
My older brother keeps telling people his favourite number is 69
user
How much does that bug you?
assistant
A lot. I don't get why he thinks it's so funny?
user
Are you having trouble explaining to him why you don't find it funny?
assistant
Yes, I don't understand why 69 is such a funny number. Do you?
user