"He has saved me a place."
Translation:Lui mi ha tenuto un posto.
"tenuto" inflects according to the object of the sentence: it is masculine because "posto" is masculine. "Mi" is not the direct object of the verb (it means "for me") so it doesn't influence the participle's gender.
Also, the participle often doesn't inflect when it is part of transitive, active, present perfect verb form: "Ero stanca, lui mi ha tenuto sveglia"/"ero stanca, lui mi ha tenuta sveglia" (I was tired, he kept me awake) are both used in Italian.
The gender distinction is not optional with passive forms or intransitive verbs (essentially, those that have "essere" as auxiliary verb) "She went home" = "è andata a casa", "This house was built 20 years ago" = "Questa casa è stata costruita 20 anni fa".
I had not noticed the fact that "mi" was the indirect object, hence my confusion. It is perhaps obvious, since "posto" is clearly the direct object, but I should have taken more time to think through the sentence in English as "He has saved a place FOR me", such that it would be obvious that "me" was dative.
I see. Thank you for taking the time to answer. Just one tiny niggling thing (and as a possible return favour) in English it would be "he kept a place for me" rather then "to me" (prepositions are a useful nuisance, since as useful they might be, they are also often false friends when you try and translate them directly from one language to another - as I am finding to my cost, time and again)