And tsurikomi-goshi...I never thought of that. That's actually a pretty good idea, especially since down the road when I get more proficient at it I can just switch to the sode variation for maximum airtime. Thanks a lot!
Sode TSG is a great throw, but don't underestimate plain old TSG. Nobody does it anymore, but it's still a great throw.
In ai yotsu, you can go Kosoto Gake to the lead leg, then left Sode TSG.
In any case, practicing tsurikomi should help general forward throw practice.
I'm going to disagree slightly with Ben, shock horror!
Whilst I agree that you should be practicing the tsurikomi action and mastering it and that tsurikomi goshi is a great throw for teaching core skills like elbow management, getting hips low etc... and is a big throw when done properly.
If you try and have tsurikomi goshi as you principal forward throw, you're going to be very frustrated for a very long time, because its a really hard throw to get especially seeing as you're coming to it 4 years in and a brown belt rather than as a rank beginner. Thus having many more bad habits that will need to be ironed out before you'll see progress with it.
Although given your should problems I'm hesitant to suggest Morote seoi nage or Tai otoshi. Perhaps sticking with Ippon seoi nage would be best.
Part of the problem with your Uchi Mata and Harai Goshi was I'm sure your lack of tsurikomi skill. You might find that different after getting TSG down.
If he gets the TSG down, then moving to Seoi Nage or even Tai Otoshi isn't out of the question. Use the TSG as a launching platform to get basic skills as you noted up to par, then branch out from there. At three practices a week, 6 months of TSG would go a long way, if he has someone who can teach it and give good guidance. One year would be more like it, though.
So, it's too early for him to specialize in a forward throw. TSG offers the opportunity to get "caught up" so to speak on fundamentals, then use those core skills to try other forward throws.
Plus, Osoto Gari to TSG is a piece of cake.
I suggest you start with a combination of static 'half turns' as we call them over here, like so:
I've picked this one, because its in Frog and you'll be able to get more out of it:
For moving tsurikomi practice:
What I'm suggesting is striking a trade off between development and achievement. As you say to progress with TSG would take a minimum of 6-12 months and that's just to see some decent moving uchikomi/nagekomi.
What I would 'proscribe' is a combination of tsurikomi drills, as outlined above, whilst focusing on his Ippon seoi nage. That way you build the core skills and he gets to achieve much quicker.
I think that's a more rewarding learning process, because otherwise life is just going to be very frustrating as he puts all the work in for TSG and sees no throws come from it in randori. Although the benefits of tsurikomi practice will tranfer over to other throws in subtle ways.