Byetta should be taken prior to meals. It works by slowing down the digestion of food and thus allowing the pancreas to keep up with insulin production. It also decreases appetite but isn't really going to be effective if you've eatan an entire meal, take the med and then dont feel like eating. By 90 min after a meal a lof of your digestion is going to already be done and your blood sugar will have already spiked. And it won't work like insulin to lower blood sugars when your blood sugar is already elevated. It's supposed to be taken before breakfast and dinner. I've also heard of people taking it before lunch if they are on the maximum dose and still have a spke after lunch time.
If the reason why the injection time varies is because his wife isnt always available to give the injection, well then time for him to be a grown up and learn to give himself the shot. The needles are tiny, the same needles used for insulin injections and if a child can give themselves an injection so can a grown man. If he doesn't inject himself because of disability, then she needs to be a better caregiver or provide someone that will give him his meds as directed. Site rotation is also important to avoid scar tissue build up in the injection area, and my feelings on this one kind of go along with my feelings on the whole caregiver thing....
I'm sure others will disagree with me.
good luck