I wouldn't worry about pushing the academics so much over the summer ... it sounds like you are doing everything right in that area already

A 4-year old really doesn't need to be sitting down and doing academic work but having experiences with academics is great (being read to, counting peas on the dinner plate, finding shapes on a walk around the neighborhood, etc...). As for the sensory issues here are some ideas:
* ball pits - you can get a cheap kiddie pool and a bag of the balls on their own and make one rather inexpensively. This develops spacial awareness, and helps with sensitivity to touch.
* sensory tables (or heck, dump out the balls from the ball pit and make it a full body sensory experience

) - for this a large plastic tub that is fairly shallow is perfect. Fill it with different things for the child to explore. Start with dry and move to wet. Rice, pasta (use varied shapes), shredded news paper, packing peanuts, and legos can be great dry items. Shaving cream, paint, cooked rice, pudding, clay, apple sauce, and goop (a 1 to 1 mix of corn starch and water and you can add coloring if you want) can be great wet items. Let the child touch and play with the materials. You can provide some sand toy type things for dry play and paint brush type toys for wet play, but the goal is to get the child to touch all the different textures. If he is really adverse to wet things, start with dry (rice works well as a starter) then add a little water to it. Keep adding until the child gets used to the wet feel.
* sensory input toys: vibrating pillows/toys, soft bristled hairbrushes, bumpy balls, massage tools, "koosh" style balls, .... anything with an unusual or interesting texture. Put all the toys in a box and when you get time to play pull it out and sit with the child. Encourage him to explore the items and try to get him to let you put them on different body parts (hands, feet, legs, arms, and back - in that order if he has an aversion). Brushing lightly against the skin with a soft bristled brush can be calming to many sensory craving children. Using a brush that is soft, but doesn't give too easily (think soft hair brush over paint brush) before bed or before a time the child needs to stay calm (like before waiting in the dr's office or going to a church service) can be helpful.
* large motor time - provide lots of opportunities for the child to run, jump, etc... climbing toys for the back yard are great, sit-n-spin for indoors is great too (esp for rainy days), and hopping balls (the kind with handles that you sit on and bounce) are all good for kids with sensory cravings. Try to alternate activities between high energy and low energy activities. Going at "full speed" all day can actually make it harder for the child to learn how to wind down, but not getting enough "full speed" time will make it hard for the child to handle calmer activities.
* music - use music to give him a sense of the expectations. Quiet, calm classical music for meal times or rest times. Loud, rock style music for high activity times. Experiment with lots of different music styles. Sing (even if you don't think you are good at it) as much as possible and encourage your son to sing as well. Make up little songs that can help him remember calming techniques and other behavioral things. For example: my daughter Arianna is pretty likely to hit first ask questions later. We have taught her a little song "I can breathe, I can breathe, and fold my hands, and fold my hands, then i will think, then I will think, what to do next, what to do next." (I can't remember the name of the song I set it too LOL). It has taken a lot of singing that little song to get her to use it, but now I hear her singing it when she is getting upset with her sister. And when I hear it, I know she is needing help navigating a situation. In addition to behavior modification, the singing can help a lot with speech issues too! Sometimes it is easier for a child (or adult) to sing a word than to say it. Also you can set sounds he is having trouble with to songs too. We sing a little ch ch choo choo song that I made up to help my daughter Serenity get her ch sound worked out.
I hope this helps and that you made it to the end of my novel LOL