I've just re-read the whole story (It isn't very long, as it's part of a collection of very brief situations mirroring the Twelve Labours of Hercules) The lad in question represented Adonis (!) although he was actually a car mechanic, who had much pleased the great detective by diagnosing a problem with his car.

The fact is that education in the twenties for the 'lower classes' was based on the three 'r's, and certainly did not include a foreign language, either French or anything else. People had no idea how to pronounce foreign names, and indeed were suspicious of 'foreigners' in general. The young man said he'd read about Poirot, and the clever things he had done (his words). So he was improvising how to say the name from the written word.

Poirot was in fact very kind to the lad, who had fallen in love with a mysterious young lady. I won't spoil the ending, but he travelled across Europe on behalf of the boy, and indicated that he wouldn't charge him for his services. The book says he fully appreciated the lad's beauty (!) from an aesthetic point of view. So it seems he felt compassionate towards him and wouldn't have corrected his abysmal pronunciation of his name.