Need advice

So my boyfriend has a job opportunity to go to Philadelphia and work for 3 weeks with the company he works for. Well he has never gone 3 weeks without seeing me since we started dating 5 years ago, I know we will be okay but we have an 18 month old who has NEVER gone a day without his daddy and he absolutely adores him. I know my son is going to have a very hard time so what should I do to help my son adjust better?