My son asked why we spell welcome with only one l when well has two l’s.

Excellent question - and I don’t know the answer. Hastily asked a few English teachers. One of them did some research and ventured that it was a result of olden English. Initially, these words were separate words. However, when combined with other words, they dropped one l in the process. That’s why we have words like although and altogether and welcome.

English is fascinating ;)

Later, my friend enlightened me as such:

It's nothing to do with well, it's to do with will: "come if you will" / "I wish you to come". It's Germanic in origin