have you tried rice milk? my ds started refusing EBM around that age. To be exact, he snatched the bottle out of my mom's hands and chucked it across the room with astonishing force....
but he loved rice milk and guzzled it. it sort of has a naturally sweet taste.
also, reverse cycling. my ds did this and only took like 2 bottles a day and nursed once (on my lunch) and then made up for ti by nursing constantly once I got home. I would try going back gradually...a couple hours first, then 4 hours, then 6, etc.
I wouldn't call it super nutritious, but if a couple bottles of it will hold him over in between solid meals while you're gone, it's worth a try.
also is that the only solid food he'll eat? I know they say baby food is supposed to be bland, but I always seasoned ds' food just like ours, and never used jar stuff, and he ate it just fine, and to this day is the least picky eater I know. You might try pureeing some already-seasoned/cooked leftovers from dinner or something and seeing if he'll take that.
um.....cheerios, mum-mums, finger foods might entertain him and if you send stuff like pieces of avocado or watermelon or maybe peel some grapes, stuff like that, it would also put a little something in his stomach.
oatmeal is always a great fallback for a hungry kid, too, if he'll eat it.
Don't know if any of that helps.....I know those are mainly solids suggestions, but anything that can help keep yuo nursing would be terrific. I think the goal is just to keep him full enough to be comfortable while you're away, and he will make it up nursing when you get back. I promise

oops, insert foot in mouth....I didn't read carefully and thought you said 8 months. nevermind some of my suggestions in that case, although I think 5 months is big enough for oatmeal. and I do mean the real kind, ds never touched the baby stuff. just make it thin.