It depends on the interpretation. I honestly like the idea that Yellow Red is the canon Red, and he got Pikachu as his starter, but was able to get the Kanto starters in a similar fashion to how you would get them if you played Yellow. But, I can also see Bulbasaur being his starter since in Pokemon Adventures, he received a Bulbasaur from Professor Oak when he had to go recapture all of Oak's Pokemon that broke from the lab.
But, to be honest, I don't think there's any definite answer since he has all three Kanto Starters and Pikachu when you face him in both G/S/C and HG/SS, but only Venusaur and Pikachu in LG/PE, which is a callback to Pokemon Adventures.
So, I'm more inclined to say Bulbasaur.
I've never really seen Charmander as his starter because I've always seen that as Blue's due to the manga, but really, it just depends on your interpenetration. If you think his starter is Charmander because Red = Fire, then that's on you. Different canons take completely different approaches to which starter he gets.
For me, though, I'm going to go with "it depends". But, like the idea of either Pikachu or Bulbasaur being his starter depending on which canon we're following.