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 totally normal. Babies are often going through growth. So it could be due to many reasons or also no reason. As long as baby still has wet Diapers, active. I'd say it's okay. Baby could also be teething. It is normal for babies to drink more on some days and lesser on other days.