Spitting up need advice

I started out breastfeeding and it wasn't easy! I have to go back to work earlier than planned so I introduced the bottle to my son. He took to it well and I substituted (sp) formula until I could pump enough milk. I noticed he spits up a lot and I thought it was do too the formula. Today I tried breastmilk in a bottle and he still spit it up too. I figure maybe he is drinking too fast. The nipple on the bottle is suppose to be slow flowing and I went from burping him every 2 ounces to burping him after 1 ounce and it still didn't help. What else can I do? Does he need to sit up during and after feedings or what?