are you able to pump at least once or twice during the day? At least pump on your lunch hour... unless you can go to your son to nurse. Also what about pumping first thing in the morning, either when you get up (before the baby is up) or as soon as you get to work. You will have the most milk in the mornings.
Another option is to pump on one side while you nurse on the other while your baby is taking his first morning feeding. i also tried to keep up my supply by pumping on the weekends, at least once per day while my baby was napping.
I also nursed my baby when I dropped him off at daycare and again when I picked him up, before I put him in the car to go home. It is quite normal for a baby to cluster nurse in the evenings, which was hard when trying to make dinner and everything, but that is what we did.. we nursed almost constantly in the evening and once or twice (I think) during the night. Some babies will reverse cycle feed meaning that they take most of their milk during the evenings & nights when their mother is away during the day.
There are some things that can help with your milk supply, like eating oatmeal every day, drinking lots of water, taking fenugreek, or mother's milk tea. You can also get a prescription for Dompridon, but that's pricy
Also, you baby will soon be 4 months & can start some solids, My milk supply decreased and I choose to supplement with food during the day instead of formula. I would send a banana to school for them to mash up & feed. Or start some cereals.
Also, if you find that you have to give formula while you are working, there is no reason why you can't continue to nurse while you are with your baby. Continue to use stage1 bottles so that he has to work a bit for his dinner. That will help him to continue to nurse when you are together.