Believe it or not, putting the kiddo in a trailer is LESS work for you than having him in a bike seat behind you.

In a trailer his weight is managed by the wheels of the trailer. In a bike seat on the back of your bike, his weight is managed by you.

(imagine the difference between carrying a suitcase and pulling it behind you on wheels)

Being in a trailer is also safer for him than being on the back of your bike.

Be sure he wears a helmet!