Seems like your opponent has 2 vastly different serves, the "slow, short serve in doubles" and "a fast enough serve that if I step in before he delivers the serve he could hurt me with it." Most players have a tell on which serve they intend to hit (ball toss, starting location on the baseline, grip, where the doubles partner stands, etc...). Learn to read your opponents serve so you are not suprised by which one is coming your way.
Most players also have a favorite go-to serve (kick serve to the backhand, slice up the tee), make them beat you with something else.
Once you know the "slow, short serve" is coming, move in and pick your spot - server's feet if he is coming in, deep if he is staying back, dropshot or short angle, lob over the net players head, lob to the middle deep, your choices are endless if you are hitting from around the service line.