. I think that to invest, firstly, he needs to have a source of income
I think that you needs to be corrected here, having a source of income doesn't guarantees your abilities to invest in bitcoin on a consistent basis without problems, because their are some folks that have a source of income but still can't figure out their discretionary income, and we all know that it's through our discretionary income we invest with, which is the money left after all our basic needs have been met.
So it's discretionary income you need, in other to get started not a source of income.
and secondly, he can invest only if he has some knowledge about Bitcoin, but I would say that any person can invest if he wants and he can become prudent while investing
You don't have to make it looks complicating mate, basic knowledge on Bitcoin is all that you need to get started, along the line you can seek for more knowledge on it while investing, but waiting to know it all is a very wrong move that may delay your Bitcoin accumulation and investment journey.