I think it all depends on your situations.
If he works shifts or has changing hours then it would be more reasonable to have flexible hours. As long as he comes every week and you know which day it will be I don't see any problem. In intact families where parents work shifts the children get
used to it. You simply explain to your child he will have so many sleeps till daddy comes next, and you can make a chart on the wall and it's a great way of learning to count. My son learnt to count by counting how many sleeps till we went to the zoo, or his cousins house etc.
Also your child is young and should be seeing his dad more than once a week I feel. Is there no option for an afternoon or evening during the week too? It is important for frequent visits at younger ages to encourage the bonding process and for dad to be more than just another visitor like a grandparent or aunt/uncle.
What about nursery, is he starting that soon? Any option for his dad to drop him off or collect him from that and give him his lunch and the bring him back to you? Gives you a longer break and keeps dad involved.
Keep the hours fixed can make your life much simpler, and if your child was older and becoming anxious with infrequent visits or an unrelaibale dad then that would be different. But it seems to me like your making an issue when non exists.
Do everything to encourage contact, putting obstacles in the way and dictating when he can see his child is signs of a controlling mother who thinks they are the main parent. Children should have two equal parents imo, and should regard them both as equal, and be involved in all aspects of their lives.
Also making the day fixed might mean dad doesn't can't come every week, which would confuse your child more. I suggest you speak or write to your ex and see if he would be happy to go for a fixed day, and possibly an evening after work. Suggest it don't dictate it.
Keep up what seems like a working situation if you possibly can. It's important for your son to have a real relationship with his dad.
Best of luck
Gilly