In my experience - and I know this might go against what a lot of other BJJ teachers say - you should never block his hips with your forearm from under side control. The reasons being:
1) Advanced people will drop all their weight on your arm and your arm will get stuck to the mat and you won't be able to turn.
2) If he drops his weight on your arm, your arm will become very tired, very quickly.
3) When your arms go away from your body he can get his chest on your chest and make it even harder to get him off.
4) If you block his hip with your forearm and try to move your hips away your arm will stay where it is. This will make it difficult for you to turn onto your side fully, so he can drive with his chest against your top shoulder and push you flat on your back again.
5) There's no way you can block someone your own size or heavier from moving with your forearm against his hip.
6) You're susceptible to kesa-katame :)
Here is a very simple escape for basic side control when your opponent is on your right side that I use myself:
1) Keep your arms in tight against your body.
2) Walk your feet away
3) Bridge then drop onto your right side
4) As you drop down from your bridge your left hand should grab his right wrist (where he's blocking your hip).
5) Your right ear is against your right bicep so he can't cross-face you and put you flat on your back again
6) Keep control of his wrist so he can't run around you.
7) Walk your feet away so you're 180 degrees from your opponent or put him back in your guard.
Points 1 through 6 are all key points that must be followed for you to escape. I can't emphasise point 2 enough - you have to walk your feet away first or you'll be trying to turn *into* your opponent and it just won't work because you won't be able to shift your opponent.
There are plenty of escapes out there, but be very wary of any escape that has you trying to support your opponent's weight with your forearm against his hip.