Try having him sit on a couch and slouch back against the backrest some. For reverse cowgirl, you can sit on his lap and keep your feet on the floor. Either he spreads his legs, or you do. If you have an ottoman, he can rest his ass on it and you can straddle it with your legs, if it isn't too wide.
For regular cowgirl, leave a bit of a gap between the seat cushion of the sofa and the ottoman big enough for your legs to fit through so you can keep your feet on the floor. Put his ass on the ottoman and his back resting on the seat cushion of the sofa.
An ottoman that is not too wide is also great for sex in the sitting position where you face each other. Places your tits very conveniently right at mouth level, for most couples.