firstly ,,,stewie is showing typical mastiff behaviour,,ie hes jealous of bryan,, he sees that somebody else has mummys attention.
he still needs to be shown very firmly that you are the boss kake or he will take advantage and his behaviour will escalate,,
try going back to some very basic clicker training ie ,, click then if he sits down a treat,, click if hes quiet treat,,click if he lays down ,treat etc.. if he shows attention seeking tactics you must ignore him!!
the clicker training doesnt work on guys tho
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