It is never too early to establish a bedtime routine. Babies love consistency and predictability. To successfully implement a routine, you and your partner can decide what you both would like to be a part of the routine.
What components resonate with you?
Some activities that have been proven to be calming are giving your baby a bath, giving your baby a massage while you put lotion on his body, putting him in pajamas in a dimly lit room, reading him a book or telling him a bedtime story, and giving him a bottle or nursing him. Some parents put their baby in the crib at this point and expect him to go to sleep on his own, while others hold the baby and rock him to sleep while singing a lullaby or shushing him. This is a very personal decision.
You could place them down for the night and say a phrase, such as “night-night” to symbolize a bedtime. Once you have determined parts of your routine, then be consistent, however recognize that the routine will sometimes be disrupted if your baby is off schedule, sick, teething, or other circumstances come into play. It is nice to have the security of consistency, but he should be able to go with the flow from time to time as well.