Hi L.,
I was in exactly the same situation as you when my daughter was 4mos old. Has your pediatrician said you can introduce rice cereal yet? That is how we handled this problem. Breast milk, as you know, does not last that long in their little tummies. If you get the ok from your doc, start mixing up a little rice cereal with expressed breast milk and feed it to your son at the dinner table, while the rest of you are eating dinner. (You don't want to do this right before bed, because you don't want him to miss the regular bf session) The rice will hold him longer, but you won't have to go to formula. Make the cereal pretty runny and be patient - it's messy - they don't understand the spoon at first. Also, you might want to try having Daddy get up once in the night to help soothe him back to sleep, without food. That way, you could get a 6hr block of sleep, and your son will learn to put himself to sleep and bond more with Dad.
I hope this helps! My girls are now 4yrs and 2.5yrs, are healthy and wonderful sleepers! J.