Can boys put tampons in their anus?

Some boys do use tampons - menstruation is a function of the female body, but some people with female anatomy identify as boys - in that situation it's no different from a girl using a tampon. If a boy is biologically male then the only way he'd be able to use a tampon is if inserted anally, which would be uncomfortable and highly unhygienic - he'd risk anal infections and risk TSS.