Lebowski

my old man is 58 his knees are really stiff, sometimes he has to walk with a cane. his back is destroyed from decades of hard labor, he has a couple missing/crushed discs in his spine. he's pretty much as stiff as a board. he was really active when i was younger but in the last 5 years he has gotten pretty out of shape because he cant really move with out being in pain.

anyway I've been mountain biking a lot lately and he expressed interest in wanting to accompany me. he said he didn't care how bad it hurt him he just really wanted to ride his bike again, he used to all the time. They opened up some new public trails in the area that go through some real scenic forest areas. and he's always walking in the woods (really really slowly mind you) he's a nature freak so he really wants to do it. one of the trails is several miles long, flat and on hard pack gravel. this one i have no worries about you could practically do it on a road bike. the other trail is a pretty high-intermediate skill level, 7.5 miles, lots of ups and downs, tree roots, big rocks. its a challenging ride for me, an 18 year old in good shape. its the kinda thing that makes me wish my mountain bike had full suspension.

he took his 1996 schwinn cruiser 6 to the bike shop to be tuned up. im glad he did because i didn't want to re-grease bearings and luckily he does not trust me as his personal bike mechanic. its been sitting in the garage in disrepair for years. i tried to convince him that the bike would not do well on the trails but he's too stubborn. he thinks just because it has wheels with nobby tires that it will fare well. the thing is a tank it weighs a ton. dont get me wrong its a sweet bike and very well made, but its no trail bike. just for reference, it has 6 gears and cantilever brakes, not coasters. oh and it has those big stupid "U" shaped grandpa handle bars that will epically fail when it comes to maneuvering.

so heres my concerns:
he hasn't been on a bike in several years.
he is in bad physical shape.
his bike weighs close to 40 pounds.
his bike has no suspension
it handles very poorly offroad
did i mention it weighs more than a boulder? i doubt the folks at schwinn could have made it heavier if they tried.

am i being too concerned? i bailed a lot when i first started trail riding, if he does the same thing he's gonna be out of commission for a long long time. he's 6ft 3" 250lbs. how can i tell him that its above his skill level with out making him feel bad? i tried to tell him to train for it, do a couple weeks of riding before doing it but he just wants to jump in and do it. the only thing i can think of is getting him to ride my mountain bike and i can deal with lugging his tank of a bike around or ride my bmx bike. but i doubt he's gonna want to sit on my mtb seat that makes his bike seat look like a couch.

do you older riders have any advice?