I agree that we need to know what tent you use when going with your son, especially how much it weighs. There's no point in getting a solo tent that weighs more than the 2-person tent you already have--save your money and take the bigger tent!

In your area, for 3-season use, the tarp setup I suggested should work fine!
May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey