Ya sometimes our mothers or in law has their own ways & thinking.
Understand yr son is coming to 4mths old. If yr dear son's weight is increasing, there's no reason that he needs to start having cereal at 4mths, somemore you mentioned yr son can sleep thru the nite. The best time to start weaning is when he turn 6mths old.
And is advisable to feed cereal with a spoon directly.