I suggest you train his ID at least to Prolific. Since both RB and IS are three levels higher than ID, the ID training will go a bit faster than usual.
Also if you leave ID at the level it is now and train just IS, you will be training IS at a bit slower pace since lower ID will drag him down (elastic effect).
However, ID training will drag down SB level of four, so I'd suggest to up SB to at least 6, then ID to 11 and then continue with other inside abilities. I think SB and ID will train really fast for you, especially if you use 1-position training (SB and/or ID should somewhere probably pop even twice in a row).