Not quite the definitive answer you probably wanted. Right?
It could be. As already noted, if he's a full-time, experienced agent and if he's familiar with the communities you're looking at, those are all strong reasons for considering using him.
Still, as noted below, interview him just as you would any other agent.
You did add: "I am hesitant because he is very busy. I ask him questions through email and sometimes it takes a week to get an answer. I am afriad of finding "the one" and possibly losing it. Should there be a time liimit on how long it takes for him to get back with me?"
And that's a real red flag. A week to get an answer? I'd say (just my personal opinion here) that that's too long. A few hours, or certainly the next day, is a response time a lot of people look for. You very well could lose out on multiple properties if it takes a week to get an answer. Can you set a time limit? Sure. Discuss the subject with him. Explain your concern. Set a response time you and he are both comfortable with. Again, a few hours or the next day are not usually unreasonable.
And there's another, totally different, concern. That is, it could put stress on your relationship with him, as well as your relationship with your husband. Now, having said that, all agents are taught to solicit business from their "sphere of influence"--friends, relatives, and neighbors. So what your father-in-law is doing is what many agents do, and most are taught to do. Still, buying real estate can involve a lot of stress. Only you can answer the question of whether your relationship with your father-in-law (and potentially your husband) would be strained if problems arose.
Again, there's no clear answer to your question. You just have to weigh all the pros and cons. And, as noted, interview him along with other agents.
Hope that helps.