My boy is 5 months+, just wondering if its okay if I let him have the rusk biscuits? Those are hard ones but will soften up after a while. He also seem to be 'chewing' on his pacifier

From what you have typed, it seems like he is teething and is chewing on his pacifier to relieve his sore gums. Try giving him a chilled teething ring. It is recommended to start giving teething rusks from 6 months of age. Do supervise him when he is chewing on these rusks. http://sg.theasianparent.com/best-first-snacks-for-your-baby-shopping-guide-special/