My 3 1/2 month olds milk quantity has dropped by 70/100 ml a day. Also he doesn't take big feeds like before now a days.. Is it normal?

Yes, sometimes baby will have low appetite and take less milk on certain days. Try to monitor his or her diapers for urine and poop to ensure he is drinking enough. You will find certain days your baby will take in more milk due to growth sprut. Just like adults, sometimes we will eat more too.