Young Cat Cannot Stop Playing!

I have a young cat, 1.5yr male, who lives with two 10-yr old female cats (sisters, very close).  He gets along very well with one of the girls and they play alot, but he is pouncing on the other one (who has only just tolerated him to this point) constantly for the past month or two.

Side notes - he was my mother's cat and she passed away a couple of months ago.  He has lived with these other two cats, with no other changes in the household, almost his whole life.  Previously he and the grouchy cat (well, she is NOW) ignored each other.

I am guessing that he is still missing my mom and taking it out on the grouchy cat.  He has energy to spare and even though we do play with him alot, his attention to her seems to be on the increase.  No destruction involved, just lots of chasing and pouncing with nails withdrawn.

We are considering adopting another young cat, a female, to play with him.  Good idea/bad idea?  I could use some help.

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