Pandrade, when our ds was your ds' age we supplemented science with lots of trips to museums, keeping a wide variety of high-level books around the house that he could choose to pick up and read when and if he wanted to (he rarely did this with us watching him - if we were around, he wanted us to read to him, and we did that - a lot!). You can also find a lot of science experiments online that often only use very inexpensive items that you might already have around the house - google "science fair projects".
Re your ds' achievement in school - I'll second everything gabalyn mentioned, and add one more thing. My EG ds had a similar but slightly larger discrepancy between VIQ/PRI and PSI. His WM was slightly higher, but also lower than his PRI/VIQ scores. We knew he was danged smart going into elementary school but he never was a high achiever, and by the time he was in 2nd grade we were hearing from his teacher that he was not paying attention, daydreaming etc. She was convinced he had ADHD... but really what was happening was he had an LD. I would look very closely at what's going on at school - ie, when is he fidgeting, what does he say about school, what type of work does he dive into and complete easily vs are there types of schoolwork that he avoids or complains about? LDs can be mirrored in IQ scores when you see discrepancies such as this - I'm not saying your ds *has* an LD but only that it is one possibility for why there is such a large discrepancy. If he does have an LD, that difference in abilities might be causing him a lot of frustration and could be the reason he's not at the top of his class.
FWIW, my ds was never at the "top" of his class in elementary school - those early years were focused on learning (for us) how to best give him the accommodations and remediation he needed to compensate for his LD. Once we'd gotten through that, and once he was in middle school - he bubbled right up there to the top of his class. So I wouldn't necessarily say that it's easy to tell if the reason you're seeing him not succeed in the classroom is simply because he's so out there in ability, because he's just a quirky kid who's not driven by that high-achievement mentality, or something in his ability profile is holding him back. If you have a chance to, I'd ask the person who gave him the WISC what his/her impressions are re what might be causing the discrepancies in scores.
Re schools - I'd definitely take the scores you have and *talk* to the public HG school. See what their thoughts are, and whether or not you feel like it might be a good fit. So much depends on individual school, it's impossible for any of us who aren't in your school district to really give relevant advice about schools. I know that in our school district the HG+ school did make some accommodations in general that would have possibly helped a child who was slower in output (lower PSI) than other schools might have thought to do... otoh, there were things about our HG+ school that I felt would have been a bad fit for our ds so we chose not to go there. Instead we put our ds in an academically challenging private school where we are able to work with the school staff to make sure ds has the accommodations he needs, and where we have a very understanding staff. At the end of the day, having teachers who are willing to work with you and who care about your child sometimes means more than the label on a school, and the only way to know what's best for your child and your family is to get out there and research all the options that are available where you live.
Best wishes,
polarbear
ps - another idea re science enrichment - our ds really enjoys taking science courses through CTY. They can be very pricey, but the cost to test through the talent search is not expensive at all, and they do offer financial aid for students who qualify for their courses. I'm not trying to promote CTY - I use it as an example because it's the talent search that was promoted through our school district so that's how we landed there. I suspect you'll find the same is true of other talent search programs also (EPGY offers group options which cost around $100 for 10 months of whatever classes you want to take - if I remember correctly). Our state also has "homeschool" programs that you can take advantage of when you have your children enrolled in private school, and we've used that to help fund our ds' online enrichment courses... so dig around a bit, you might be able to find something online that he'd enjoy.
