Standalone measles vaccines were available from the 60s, then combined measles/mumps, then MMR in the late 80s, so those of us who remember having been vaccinated in the 60s/70s are right, it was just the single vaccine.
I had measles in the 70s after vaccination, but before the two doses were recommended. I remember hallucinating, vomiting constantly, huge fever, fainting on the way to the bathroom, having to be sponged to keep the fever down etc. Such a relief when the rash came out because it meant the worst of the fever was over. In bed for say a week - really lying down in bed for a week, not just taking it easy - then a long convalescence and significant weight loss. I have hearing loss in one ear though not necessarily linked to that particular childhood illness.
My parents do remember children dying, losing their vision and suffering brain damage from measles encephalitis. It was a regular part of life for them.
OP, it’s completely understandable to be worried. Don’t beat yourself up, but discuss with the GP. Your child can be monitored carefully after the vaccination. If your child is prone to febrile convulsions it’s all the more reason to have him fully vaccinated.