What a hard time for your little guy. If it were me, I would focus on everything positive and try to minimize anything negative.
Spend quality time with your little guy. One on one time with you doing things he enjoys is the best medicine. Let him pick some at-home evening activities like family game night, family movie night, family baking night, family camp out in the living room night...whatever makes him feel like the three of you are a family together. Be willing to be silly and frolick with your kids, laughter will bind you together.
Don't talk badly about dad, money issues, custody issues, etc. He is too young to understand or deal with the complexity of those problems. Focus on how he can spend time with you and he can spend time with dad. Isn't that great?!
Set boundries. It would be tempting to make him happy by overindulging him, but long term that creates more problems than it solves. Happiness comes from being with those you love, not from watching TV, owning things, or having a messy room.
Books! Anytime I have to introduce my kids to something new, I run to the library and get books on the topic. Make sure you read them ahead of time so they communicate the message you're trying to get across. We've dealt with death, dentitists, vaccinations, financial hardship, and me returning to work through reading. Just make sure it is only an occassional book on the topic, otherwise you'll drag the little guy down. Read a few fun books, then one that talks about a changing family, then another couple silly ones. That way it appears that it is just part of life to be taken in stride.
Life is such a grand adventure! Focus on the adventure and downplay the difficulties until he is old enough to understand them. Good luck! Keep your chin up and it will help your son do the same.
M.