I have four DC, two are at uni, one has finished uni, and one in Year 10 so a while to go. I too am a lone parent so can empathise.
With each of them it has been so hard but also an exciting time where you feel so proud. You really just need to get lots booked in for the period after he goes and keep yourself busy. Avoid going into his bedroom and crying (yes, I have done that many times and no it never made me feel better!). I visit them all about once a term and stay for a weekend. It is so nice when you see them in their environment, most of the time loving it and you feel so glad you have brought them up to do so well.
They do come back! DD2 is in 2nd year of uni, she is quite a 'home girl', while she loves uni and has a great and full life there, she just loves coming home for visits and to 'recharge'. She comes about every 3 weeks in terms one and two and more often in the third term as she has study leave and works best coming home for some quiet to study in. Holidays are so long, DD came home nearly a month ago now and it feels like she never left. When DD went off for first year she had a tricky freshers week, be prepared for phone calls or them being unsure of things and don't worry that this means they hate it and will never settle. DD phoned me at about 1am in tears one night saying she had no friends and wanted to come home. She hadn't met anyone she clicked with and was really missing home. I wanted to cry to her and tell her I would come and get her, I resisted, kept a positive tone and said how it will get better and she just needed a good sleep. Be prepared for moments like this! More so with daughters I expect.
Remember when you leave them, you aren't throwing them out forever. They come back and are home more than they are away!
DS lives at home as he is in uni in london and the costs got too much combined with him being on placement a lot as a medical student. There are, believe it or not, times I wished he would move out!!! He is great to have around, but as they get older and get their own lives you start to feel 'it is time' to let them fly!
DD1 graduated and is working, living in London. She pops back randomly, often without warning. Stays for weekends a lot and it doesn't really feel like she has 100% gone. I definitely haven't lost her or let go of her in any way!
When youngest goes it will be hard, very hard. Especially as by then I expect all the others will have moved out.
Anyway, sorry for the ramble about my situation. Just try and push the emotions to one side and focus on the practical things like getting him ready. Plan weekend visits, look on his uni website at all the exciting societies etc he will get involved with to remind yourself what a great opportunity this is. Most of all, as I said, keep busy running up to, and in the weeks after, he has gone.