"The man likes his trousers."
Translation:Mężczyzna lubi swoje spodnie.
Spodnie are always plural. Therefore: Mężczyźnie podobają się jego spodnie. "się" is movable, but "się podobają" sounds off to me, although I guess it's not technically wrong.
Now we have a problem. Such a sentence makes sense, but rather only if you interpret it as "The man (Adam) likes his (Mark's) trousers." For which you'd rather just say "Adam likes Mark's trousers", right?
"podobać się" is mostly for first impressions, and mostly visual ones. So "Mężczyźnie podobają się te spodnie" makes perfect sense to say when he is in H&M trying to buy some trousers. He sees them, they look cool, he wonders whether he should buy them. If he already owns them, and therefore has some... longer relation with them, they are super comfortable etc. - you should rather just go with "lubi". It's not that it's totally impossible that a Polish person will say "Podobają mi się moje spodnie", but it would be quite odd to my ear.