Is the Med school that is upcoming, going to be in your same town?
Or he will be away for that too?
It seems this cannot be avoided, him being away, per his job, then for med school.
A child will miss their parent. Him saying he does not like Daddy is just him at his young age, TRYING to express his feelings. They are not astute about emotions, they just are trying to fathom it.
It is not literal.
But YES, continue to allow your son to express himself.... boys especially need to know that. Being pent-up about his feelings, are worse. In the long run. A child needs a soft place to fall, and a shoulder to lean on and say things to. You or Daddy.
I have a friend who's Husband is always away, on work trips and overseas, and he just has LONG hours and is hardly home. They have 3 young children. They miss their Daddy. But when he is home, he is THERE and very hands on. And they communicate often. AND he stays in touch, with his Wife about EVERYTHING. Because, he knows it is stressful on his wife, too.
For them, this is just the nature of his job. But they are all, very close. And they manage. And the kids too.... for the kids, this is all they know- ie: their Daddy being away a lot for his job. But again, he is very in touch with his family and kids and wife and is very hands on. And he is a thoughtful guy.
Like the other respondent said below: if there is an end in sight, then this is all temporary. But if your Husband will always be away, and per med school... then I don't know... your son, like many children, will miss their parent, and they don't know how the world works, nor that their parent is gone so much.
But you/husband need to sock away money. That is the immediate at hand. Its hard.
Besides just doing Skype or Husband playing with your son when he is home: your Husband can MAIL him cute little letters. Kids like to get mail. Even drawings in crayon that your Husband makes. Or a note saying "I love you, you are my special boy...." on it and mail it to your son. Any parent can find time to do that. Your son will appreciate it and feel "special."
And it doesn't cost, much... but will be a lot in terms of emotional well being for your son, and him feeling that his Daddy is thinking of him even if he is far away.
Simple things, but meaningful things, for a young child.
And, I am sure you are talking with your son, "explaining" that Daddy is working but loves him. AND also, your Husband can, talk with your son too... to give him solace about him being away in person, but close in heart etc. Kids, benefit by being talked with, about the situation. In kid terms that they can understand. It can really help.
And validating their feelings, too.
ie: when my kids would say that they miss Daddy, because he was working so much and not home... I would tell them "Yes, I know you miss him. I miss him too... its hard..." and hug my kids. Just validating them and showing them YOU understand, makes a big difference. Versus just trying to change their feelings or dissuading them.