That is very sweet of him, possibly relating towards you with a protective brotherly or a possessive fatherly instinct. I am sure he will understand as he grows up and you can explain it as well especially with regard to his father whom he has to learn to see as a nourisher and nurturer, a fosteror and protector rather than a competitor. May be you ask your husband to spend more time with him to develop a friendly bond.
Affectionately,
Alok Da