Tiggs, I've been in the same position as you. I have a full-time job and also work evenings and weekends on top. I have no choice. OH has a full-time job, is doing all the renovations on the house plus all the cleaning, laundry, etc, and now he is doing overtime himself (we have no choice). There are times when we can't stop and play with him and so he will sit beside me at the table on his tablet (at least he's doing puzzles and mental agility tasks). I got criticised for that on here by those who would pat their friends on the back for the very same thing...
At least OH and I try to juggle things between us. You are single so your situation is worse than ours. You are doing your best to raise your son and provide for him.
Why not spend 5 minutes tomorrow having a chat to him and apologise and explain that for the next week or so you will have to concentrate on some really important stuff but you will make it up to him at the soonest opportunity and ask him to have a good think and decide what he would like to do as a special treat.
When you look at some of the tragic news stories in the last few weeks, you will realise what brilliant mums we are!