So my dad has been trying to get into the whole cycling thing but he's recently faced a problem. He put on Michelin City Tires on his MTB to get rid of the knobbys. He rides an older (~2005) MTB with 26 tires. At first they were fine and he would go out riding at least three to four times a week and enjoying it. All was swell. Then, he got a huge spurt of flat tires. First started off once every other ride but then it would be multiple in the same ride. He quickly got frustrated and gave up. Now, any time he tries to ride, he get's a flat tire. Yesterday, he tired commuting to work (which is only 5 miles) and he got 2 flats on the way home and had to walk about 3 miles. He's a big guy, don't know his exact weight but at least 260. He doesn't inflate his tires before every ride even though I tell him to, so pinch flats might be a reason. But I rode with him once and inflated his tires and he still got 2 flats in 40 miles. I'm thinking either the tires need to be replaced with something more heavy duty. Or there's something lodges in the rims. I really don't know. And the tires have been checked for glass and everything each time before a new tube is installed.
Thinking this would be an appropriate place to ask as there are a lot of people with experience.