I can't say one way or another which is best (since he has a coach I would say that would be a great source for info), but I can relate what my son is doing fwiw.

My son is also 12. However he only just started bowling in January of this year (age 11). He suddenly got very interested so I got him into the league and picked up his first ball. Went 10 to be conservative; standard grip. Standard grip lasted about a week or two. He had no desire to throw a straight ball and was spinning it. No sense doing the right thing the wrong way so had it redrilled for fingertip. And he has been on a good trajectory ever since.

He also takes lessons (BowelrX....shameful plug), and I would say form wise he's better than me (release not so much). He turned 12 in July and for birthday we moved him up to 12. We (coach, driller, and I) did discuss 13 as possible, but decided to stay conservative not knowing how he'd handle the 20% jump in weight. Well he did great. And his scores continued to improve (nice leap in average actually). So much so that when he won a raffle for any ball of his choice last month, we went 13 and he mixes that in successfully with his arsenal at tournaments. He just bought another ball for light conditions and that is a 13 as well.

So we are taking incremental approach at this point and it's working well. It was a big leap from 10-12 but taking it gradually from here on out. Downside? It's going to be more expensive.  Sure by next summers birthday we will be looking for 14 pounders. But in my view it allows him to maintain consistent approach by making step by step increases in weight. YMMV.

As far as fingertip goes, Tom used to want to wait until boys had went thru most of their growth spurt and the hand had developed most of it's strength. That was generally around 14.

But Justin wanted to go to fingertip around 12 and he relented and because of that several others wanted to go fingertip. He pretty much decided that going earlier wasn't an issue.

In a lot of ways the fingertip grip is easier on the hand because you don't have as much to clear off your hand. The fingertip fit is almost always a superior fit.

As far as weight, we used the various weight measuring balls and looked to see if the shoulder dropped. If not and ball speed wasn't affected to badly then we would go up.

14 makes a big difference in carry much more so than going to 15 or 16. In the hard rubber, plastic days 16 was need but not today.

I'll add this.

I believe fingertip fits are more sensitive to span changes than conventional grip. Kids in the 12 to 14 range will often grow so fast that they may need to lengthen their span every 6 months or so with a fingertip fit.