Oh I sympathize my DD (Also first) went through this at 12/13 mo, she is 15mo and is now beginning to understand the word no.
I did used to let her have what she wanted when she began to kick off, but I have stopped that as I didn't want her to be 'spoilt' IYSWIM.
The tantrums that ensue are not nearly as bad as you would have thought.
DD had a massive (the biggest yet) tantrum the other day as I wouldn't let her into the kitchen it lasted about an hour, I couldn't help her at the start with distraction so I just let her get on with it. Once she had finished she came over to me for a hug
He is not too young to get that message yet, he will understand that if I cry or kick off mummy will give me what I want anyway. DC even of this age are very intelligent. Please persevere with the 'no', but do pick your battles.
It is hard, but DH and I have persevered with DD, as have grandparents (my dad is ace for this) and it seems to be having the desired effect.
If you do anything, try distraction all the way, if your DS wants something you do not want him to have try distraction with a toy or some music etc. Someone on here told me to have a toy they really like but don't have often to use for distraction. We use singing with DD, she love row row row your boat and all the actions, and all is forgotten.
Good luck.