My Sims is also a scientist (at the top level). She attended the Uni by using up her paid leave. She already graduated & now, she's earning more than 15k per day. Since the term is every 5 days (7 days including the weekend), you can actually have him attends the class either alternately using his leaves or just take the unpaid leaves per term. Since they need to have at least 12 credits to graduate, you can actually either have 3 weeks or 4 weeks of Uni time depending on the vacation leaves that your Sims have. Else, you can always re-enrol the next term after you have enough leave to take.
Now, I'm planning to get her another degree at a different Uni. :D