To begin the diaper discipline with your 9 year old boy you should sit down with him and explain to him that since he's acting up then he will wear diapers when at home and when not in school.
Explain to him that you love him and you're doing this for his own good.
If he still does not cooperate then you may need to spank him to get him to comply.
Since he's wetting his pants on purpose then you should tell him that since he can't keep his pants dry he will need to wear diapers.
Lock the bathroom so he cannot go to the toilet.
Since your 9 year old son weighs 45 lbs then he should be able to fit into a Pampers Baby Dry size 6 diaper, Pampers Cruisers size 7 diaper, Luvs size 6 diaper or even a Gentle Steps size 7 diaper.
Putting your son back in diapers will make him eventually behave as long as you keep him in the diapers and let him know that you're in charge which includes changing his diapers.