Hyperactive 20 month old

I am having some trouble with my DS2 at bedtime. Nap time he isn't too bad - just when he goes to bed at night. We have had the same routine since he was very young and he used to be fine but lately he is getting worse.

He just wont calm down. We give him dinner and then a bath and once he's out of the bath he rus into his room and then the struggle begins. Getting him dressed always ends in screaming, but then he calms down while we have a few stories with his brother, 4 whom he shares a room with (not by choice!) and he's usually not too bad for the stories either. Then we try to calm him down and get him to lie down on the bed. The lights are dimmed and I have tried relaxing music, massage, and calm, quiet talking but he doesn't seem to respond. He will not lie down at all and cries and kicks and uses anything to keep us in the room - usually he keeps asking for water (but doesn't actually drink any) and then it ends with us yelling at him to sit still on the bed and then we leave with him screaming which upsets everyone - including his brother who it really isn't fair on.

The crying usually goes on for about 15 minutes and we go in and out every couple of minutes to try to sooth him so that our eldest can get to sleep (he gets quite concerened when we all get angry and DS is screaming).

Anyone have any ideas? I am about to give birth to our third any day now and really need to get him settled in a nice routine so that we don't both dread putting the kids to bed - it used to be such a nice experience! Any help appreciated...