You've lost control, and it's time to reassert yourself as the mom. If it's bedtime, we go to bed. The only time one may get up is to use the bathroom. Otherwise, lunch is over, it's not TV time, and Mom needs her nap, too. So go to bed. He has just figured out how to string you along and will continue to do so as long as he gets something out of it. Put your foot down kindly but firmly, put him in bed and shut the door. Each time he gets up (which he will) tell him no, it's naptime and put him back to bed. If he fights you physically or tantrums, give him a swat on the bottom and shut the door. The less said the better - you've made your point so don't belabor it with too much talk. And stay calm. The biggest mistake parents make is taking their child's misbehavior personally and arguing with the kid. Since he already knows the naptiem drill it won't take long to get him back in the swing. But, since you have opened the door to getting other things (snack, hugs, TV) he's realized there is life on the other side of the bedroom door and he doesn't want to miss it! So remove the temptations (say no to everything he requests except the bathroom) and he'll stay in bed. Also realize that this test will come around again, perhaps in another form, so be watching for it and stand firm.
As kids develop internal control they can learn to put up with hearing "life sounds" going on throughout the house, but allow themselves to sleep anyway. This is better than having to keep a silent house during naptime. Besides, with your new little one coming, you need the eldest well trained and helping you, not hindering!
SAHM of seven, 23yr - 21 mos. My youngest says "night night" and pulls me into the bedroom to put him down at naptime. He's already learned to love his nap. Without it he's a cranky pants!