"He says that he likes those pants."
Translation:On mówi, że lubi tamte spodnie.
If you put it in the right grammatical form, it makes sense - especially if he is in the shop wondering whether to buy those pants.
"spodnie" are plurale tantum, they don't have a singular form. Just like pants/trousers. So it's "podobają mu się" (they are pleasing to him). I think that "mu" (to him) has to come in-between 'podobają' and 'się', I don't see a better position for it.