By the age of four most children are toilet trained, but it's not alarming if a child isn't trained at that age. By five, there's more reason to be concerned.
When there is a marked delay in an area of development (e.g., speech/language), it can be an isolated delay, but there can also be additional areas of delay. It would make sense to have an evaluation by a developmental pediatrician to investigate this boy's status in the various spheres of development.