At 5, you should be able to feel pretty sure about what you are dealing with; he's about the equivalent of a human college student as far as maturity (still capable of having "young and stupid" moments, but if he had an assertive and aggressive personality, you'd certainly know it by now). Sounds like he's a pretty mellow little guy.
As far as the jennet goes, it may be socialization, or it may just be who she is. The donkey/mule hatred for dogs is legendary, yet whether it's all dogs, just unfamiliar dogs, or they really don't seem to give a hoot one way or the other, is to some degree an individual thing. I've talked to people who have had a donkey/mule for years (sometimes several) and never had a problem (some didn't even know that there might be a problem), and I've seen mules that you can't even have a dog on a lead near, because they will go after it, even with the owner right there.
As far as the jennet goes, it may be socialization, or it may just be who she is. The donkey/mule hatred for dogs is legendary, yet whether it's all dogs, just unfamiliar dogs, or they really don't seem to give a hoot one way or the other, is to some degree an individual thing. I've talked to people who have had a donkey/mule for years (sometimes several) and never had a problem (some didn't even know that there might be a problem), and I've seen mules that you can't even have a dog on a lead near, because they will go after it, even with the owner right there.