So long as the Baby has been fed by their Pigeon Parents uteill say day five or so, then, so long as we, as surrogate parents, provide adequate nutrition and feeding in general, the Baby-unto-Adult will be on parr with any Wild Pigeon as far as their growth and development.
If we are starting with a just hatched Neonate, or very very young Neonates less than say four or five days old, it might be more dicey, as we have no way to provide the antibodies or other subtle and important materials the natural Pigeon Milk from their Pigeon Parents provide.
Generally, out of prudence, or caution of Crop Problems, people who are raising a Pigeon Baby will not feed as generously or as often as the Pigeon-Parents would...so, people raised Babys may develop more slowly or be smaller in size as they develop.
It should be fine to leave him with Seeds and Water by day...so long as he is safe from dangers of other people, dogs, cats or whatever.
And, allow him all the explorations you can, and exercise, of course.
Do you wish to release him once he is old enough?
If so, start now and waste no time, in letting him peck and spend time with a feral flock as they graze, as you supervise from a short distance of course...once he is flying, it will be too late to do so, and you will have missed the time window he will have needed, for his gaining his socialization skills and wild-modes-awareness he needs to acquire from them, which you do not have to offer...this is an important learning time for them, which their Pigeon Parents do once the youngster is flying...but, which we have to do before they are flying.