Longview is a relatively small town, probably 35 miles east of Tyler. I've been there quite a few times and have been in that Waffle House before. You will see all kinds of people there. Considering all, I don't think it's unusual they wouldn't notice his truck there for several days. I don't think the employees would even know what he drives unless he had parked in front sometimes and they noticed him getting out. Too many trucks that look similar, and every guy in east Tx drives a pickup, and some women do. There may be two waitresses on duty at one time, and a cook and maybe a dishwasher. Busy times they might have three waitresses, but the place is not very big, best I can remember. If he's a regular, they may know him by sight, but not really know much about him. This is MOO, of course.
It really sounds like he may have been targeted by some people that he may have known slightly but not well enough. Maybe he went willingly and then they took him somewhere and hurt or killed him. Whatever, it doesn't sound good at all. He is probably on disability, and may have just gotten his check and cashed it. Could have been "flashing" some money in the restaurant, not realizing that was dangerous. People with mental disabilities don't think of danger, they like everybody and trust everybody.
I feel so sorry for his mom and his brother.