I fully agree with @Buzz.
There are some answers on this topic at Teaching a third language to a bilingual toddler which I think will be useful for you.
Which language is more difficult is somewhat subjective, for example you and I would probably perceive Chinese as being more difficult than English, but for a child growing up in China with parents who consistently speak to him in Chinese, English would be perceived as more difficult! However, I don't want to express an opinion about Hindi specifically, since I know nothing about it.
I agree that not having many people to communicate with in a target language, does makes things more challenging. Perhaps a little less so in the early years, but this can become more of a problem after beginning school.
Here's how your husband can elicit some utterances in Hindi. First, pick some easy to pronounce words that express concepts he's interested in. Point to it (in real life or in a book). Ask (in Hindi), How does Daddy say it?
This is a tried and true trick! It's a little early right now to use it, and also, I need to warn you that this is a trick you don't want to over-use!
Now, here's another technique. Say your son is talking on the phone or over Skype with one of your in-laws. In the early years, chances are good he's going to mix his two languages. That's okay. You just hover nearby and every time he says an English word, you prompt him with the Hindi word. If he manages to repeat it, he will get some lovely positive feedback from the other end of the phone line. You'll probably need to play this assistive role for some time. The other thing you can do to facilitate these phone calls is to suggest topics for conversation. There are two ways you can do this:
if the conversation starts to flag, but you think your son still has some phone energy left, suggest a topic to him. For example, say the two of you went to the park in the morning and saw a duck, and he was pretty excited about the duck, and suppose he knows the word in Hindi for duck. (Or you cheated a little and looked it up, or called your husband at work to ask for the vocabulary items you would need for your phone call.) Back to the little pause in the conversation. At this point, you just say to your son one word, [duck] (in Hindi). That will get him off and running, hopefully. He will say [duck]! Your mother-in-law (or whoever) will repeat the word, excitedly, and there's the feedback loop. Or if she didn't understand him, you lean a little closer to the phone, and repeat the word more clearly, or make a simple sentence (Pidgin Hindi is okay for a doting grandmother), such as "Saw duck in park!"
Give the relative a heads-up ahead of time, by suggesting a topic of conversation. Make a short private phone call and say, "Ask about the park." And so on. You can prepare yourself a cheat sheet (phonetic if necessary).
If your in-laws are not monolingual, it would be very helpful if you could get them to refrain from speaking any English to your son.
It would be great if you could find or form a Hindi play group. If it meets at a time when your husband can't take your son, that's okay, you can take him. The other parents will admire your support for your son's Hindi, and will understand if you are rather quiet.
am hoping that my Hindi will also improve as I (hopefully) hear them speak to each other.
Yes, this does happen! Especially around the dinner table.
Oh, one more technique to share. When your husband reads your son a story, it can be a picture book written in English (or any other language). Your husband should simply come up with his own text, in Hindi, to go with the story. His text absolutely does not need to be a precise translation of the original text. This approach will work until your son gets to the level of books that have a whole paragraph per page.
