Looking at the current line up of the walmart bikes, it looks like the best deal they have is either the Schwinn cruiser that someone else recommended, or the Schwinn aluminum hardtail. I haven't seen the cruiser, but I recommended the hardtail to my 13 year brother ($140). The brakes are decent, as are the components. I don't like the shifters or the pedals very much and I can't speak for the quality of the metallurgy, but it will suffice for him until he's ready for the capability that a real bike will give him. I helped him to pick the least touched looking one off of the floor and helped him make the necessary adjustments before I let him ride more than enough to ensure there was no damage.
Quick story - I worked at Sears for the last couple of months that bikes were sold there. Someone bought one for his son, then rode it out into the parking lot without even looking at it or checking torque on the wheelnuts (something I do even with LBS bikes). He starts jumping curbs with it and guess what... the front wheel fell off. He hit his chin on the pavement. He called 3 MONTHS later to complain and say he wanted to sue.