As peachey and Jo*mar have stated, explaining your head condition is important. Another important thing is to respond quickly in pain. He needs to recognize his error immediately. His memory is not long lasting. He will need many repeats of this 'Ouch, you hurt Daddy's head.' If head butting is a common behavior, it has to be stopped.
A 14 or 16 month old is old enough to understand proper behavior and play. If you need to correct him, you need to stop him and be sure he is looking at your eyes. Hold him firmly by the shoulders and say, "Look at Daddy. You will be punished if you hit anybody with your head." Be sure he is continuously looking at your eyes so he sees the importance of what you are saying. Tara needs to be on board with this format.
If he can talk well enough, ask him to say what he did wrong. This is additional reinforcement. Keep in mind that his memory is short and he needs consistent repetition of a directive for it to make a difference. He needs to be given a short leash. No "This is the third time I have told you this" before a repercussion. He will be learning that No does not mean No until it is said three times. Use a firm controlled voice. Do not respond with an angry voice. He will shut down his understanding and just try to defend himself from the anger.
Ask him if he is sorry. When he says he is, say you forgive him and give him a firm hug. You may even ask him to rub or kiss your sore spot. It makes the injury a reality to him.
The concept we learned raising kids was simple. As adults, we have more patience to outlast their misbehavior than they have energy to be ornery.
If you have ever raised a puppy, the same techniques work there too. Both have minds that are learning behavior patterns and cause and effect.
This new relationship of Caleb taking care of Daddy will be a great accomplishment for you both. Be patient and consistent and expect him to get it.
In my experience taking care of others children from creepers to two year olds, this works well, even when they have different systems at home.
My best to you.
Mark in Idaho
58 years old, retired due to disability, married 33 years, father of three, grandfather of four, Suffered a serious concussion at 10 years old (1965) stopped most driving after last concussion at 46 years old (2001), Post Concussion Syndrome/Multiple Concussion/Impact Syndrome with PTSD, immediate and short term visual and auditory memory problems, slowed processing speed, visual and auditory processing difficulties, insomnia, absence seizures, OCD, 14 concussions since first concussion at 8 years old, Taking paroxetine for 14 years and gabapentin for 12 years. Added L-Tryptophan and successfully stopped all paroxetine after 3 months of tapering down
"Be Still and Know That I am God" Psalm 46:10