You can give formula  milk if is problem with lactation. Always consult a doctor before satrting formula milk If one milk product is not acceptable  by baby you can change the brand. Some times it takes time to adapt the transition from breast milk to formula milk

Try to change the semilac to aptamil gold and see meanwhile you express breast milk with the help of breast pump and give

Yes ,you can give formula milk

Try to give direct breast feeding Use breast pump to eject milk

Yes you can give formula

Since your baby is used to sucking and feeding your milk since last 3.5 months, he won’t like the taste and mode of feeding formula I would suggest you to take measures to optimise your breastfeeding rather than giving formula, such as 1. Drink 2-3 liters of water per day 2. Feed more at night 3. Eat nutritious food 4. Do not stress yourself How are you giving the formula? Via paladai or spoon or bottle? Bottle feeding and breastfeeding mechanisms are different, it will be difficult for a breastfed baby to accept bottle feeding Kindly consult for further assistance