Just looking this up for you.....Peter Gurney says to check the sow milk has come in by gently squeezing her nipples just to be sure milk has cme to both. He also says never to feed a baby guinea pig by syringe as if too much food goes into their mouths some can go into their lungs. A teaspoon is best. He recommends using a nonflavoured fortified convalesent drink. The pet shop near me sell powdered milk for this purpose so you maybe albe to get something simular if any shops are open yet. I`ve also read that people use vegtable baby foods to feed guinea pigs how aren`t eating(the puree first foods) These maybe easier for you to buy right now?
Hope that will help. Good luck, hope he`ll be ok