Your Pediatrician is correct. For the first year of life, a baby's primary source of nutrition if from breastmilk/formula. Don't replace solids for this. Also, giving solids first, before nursing, is primarily done to "wean" from breast.
Perhaps, try different bottle/nipples to see if he will take that, when you are at work. I love the "MAM" brand bottles. You can find it at Amazon.com Here is the link:
His weight probably dropped because he is not drinking any breastmilk/formula when you are at work. IF he is losing weight...it could also mean that PERHAPS you don't have enough milk and he does not have enough intake. Intake & calorie intake directly corresponds to a baby's weight gain. I would check on this.
ALso, make sure that when he is nursing, that you nurse him long wnough so that he reaches the "hind milk." About 15 minutes of longer at each breast.
Nurse him on demand when you are home, this will also stimulate your milk production, and/or maintain it.
Nursing from breast and drinking from a bottle, requires 2 different "sucking" techniques. A bottle is just gravity...but nursing from breast requires muscle and tongue/mouth coordination. Some babies get nipple confusion. Some just may not know how to drink from both. Make EXTRA sure, that your baby is nursing correctly, is latched on and actively nursing...some babies may "look" like they are suckling, but are not in fact nursing correctly. If need be, consult a Lactation specialist to evaluate this.
Ask your Pediatrician, if you can give your baby Pediasure. This is a supplemental drink. It's usually for Toddlers, but perhaps there is a "baby" version.
How many hours does he go WITHOUT drinking any breastmilk/formula? Remember, a baby can't go all day without nursing/drinking. BUT don't fill him up with water either.
Since he is not drinking sufficiently, make sure that he is not getting dehydrated. If he is dehydrated, you need to see your Pediatrician. For Dehydration symptoms in babies click this link:
Make sure the pumped breastmilk/formula you give him is warm. Maybe he doesn't like it cold?
Your baby losing weight/growth is a real concern. What did your Pediatrician recommend? You need to ask her/him. And, what is the solution?
I really wish you the best. take care I hope this helped,