Help needed! Breastfeeding to formula.

[I Love Lucy]

We've been having some issues with DS's weight for awhile though at his 9 month check up last night we discovered that he had lost weight rather than gained. My supply must be even lower than I thought since becoming pregnant so I now need to offer formula. We're struggling with it as DS seems to think bottles and sippy cups are more so toys than a source of food. When he's not shaking them around or chewing on the nipples he pushes them away in favor of the breast. When I do manage to get him to drink some he seems to spit up a good portion of it.