。
It's only natural for Mr. Suzuki to be proud of his son.
Literal
Suzuki-Mister [subject-が] son-[honorific] [object-を] proud [in-に] feel [nominalizer-のは] reasonable [copula-だ].
もっともだ ('reasonable, natural, understandable') is a な-adjective meaning 'it makes sense, it's only natural.' Preceded by a nominalized clause (using のは as topic marker), it evaluates that clause as reasonable. The honorific 息子さん (where さん is attached to son) is the speaker showing respect to the addressee's son — standard polite usage.