IR/heat gun is highly recommended by keepers on this forum.
Analog thermometers are typically not reliable. Digital is the way to go if you don't have a heat gun. An indoor/outdoor thermometer with a probe is recommended(notably an accurite from Walmart for $12ish). Your hot spot should be 88-92. 70-75 is still cool and should get bumped up.
I would definitely add more to the enclosure for him to hide under.
He doesn't look ridiculously underfed but could use some plumping up IMO. Hard to judge size but I'd see he could easily handle a mouse when he eats. Probably 2, imo.
Note on food- mootworm recommended rat pups which is the best option for when you go to switch your snake to bigger rats, it'll be easier later. Mice are fine for now, but you will want to switch to rats soon. Worry about that once you get things even with the snake's enclosure and eating right.