Well...imo you need to put your son in a tub and leave him there for a good long time. Only keeping his water dish full and feeding him once a week. In the meantime, you need to take your BP to baseball games and out for ice cream and always tell your BP things like "hey buddy, good job on your math test last week, I'm so proud of you"
Then after a month or so, do something really big, throw your BP a birthday party, invite over a bunch of other BP's and play pin the tail on the rat and have cake. Spend numerous nights reading your BP stories as you tuck him into your previous son's bed. (make sure you fluff his pillow!) Other than that, do other father son things together, as it definitely appears that your BP has fought his way to the top slot to be your favorite child.
Halloween will be epic, as you share your BP's collected candy between the two of you...
Then, at Christmas time, help your BP write a letter to Santa, hang lights on the tree and go Caroling. Christmas morning of course...should be HUGE.
Maybe after all these things are completed, your human son will realize that living in a tub sucks and be more participative.