Final r-cluster metathesis in one child's French

My favourite 4-year-old is doing something very interesting these days with final consonant clusters in his French. Many word-final consonant clusters starting with R get metathesised: parle (speaks) becomes [palʀ] (yet parler "to speak" remains [paʀle]), tourne (turn) becomes [tunʀ], herbe (grass) becomes [ebʀ], ferme (close) becomes [femʀ]. On the other hand, "porte" (door) remains [pɔʀt]; regarde (look!) [ʀəgaʀd]; "force" (strength) [fɔʀs]; "mars" (March) [maʀs], "parc" (park) [paʀk]. Presumably the phenomenon is related to sonority: {l, n, m, b} metathesise, {t, d, s, k} do not. But French allows word-final consonant clusters with falling or rising sonority, and he has no trouble with words like "monstre" (monster) [mõstʀ]. Any idea if this is typical in French first language acquisition?

Nothing of the sort happens in his English or his Arabic. Then again, his English is non-rhotic anyway for some reason, and in Arabic he pronounces /r/ as [ʕ]; French is the only one of his languages where he's got the pronunciation of rhotics more or less sorted.