Jaison (usually spelled Jason) is not really a Christian name. It's a name from Greek mythology, and since no one really believes in Greek mythology anymore, it's fairly religiously neutral. I just looked up the name on behindthename.com, and apparently it does briefly appear in the Christian Bible, but no one that I know associates this name with Christianity.
I don't think it sounds very good with Ishaan, though. You might try looking for a name that is three or four syllables long or that does not end in N.
Here are some suggestions. Some of these have been worn by famous Christians but do not have Christian roots:
Ishaan Alexander
Ishaan Maximilian
Ishaan Anthony
Ishaan Darius (a name used by more than one religion)
Ishaan Christopher (okay, so it's Christian, but I love the way it sounds)
Ishaan Julius
Ishaan Martin
Ishaan Theodore (used by Christians and non-Christians)
Ishaan Vincent
Ishaan Valentine
Ishaan Frederick
Ishaan William
Ishaan Henry
Ishaan Xavier (okay, so this one is connected to a Catholic saint, but it sounds really cool!)
Is this child growing up in India or in the U.S.? If he will be growing up in the U.S., choosing a name that comes from the Bible may still sound very religiously neutral to most people. That's because so many names that are common among English-speakers originally came from the Bible, but no one really considers them "Christian" names anymore. Choices like James, John, Michael, Zachary, and David are used extensively by Christian families and non-Christian families.