Commas can be used to create effect. However, in this case, I don't believe the comma is necessary.
- He was getting tired of eating sandwiches! It turned out his father was getting tired of eating sandwiches too, so they both went down to the beach, where Jamal lived with his mother.
The following is an example of it trimmed down:
- He was getting tired of eating sandwiches! It turned out his father was tired of eating them too so, they both went down to the beach where Jamal lived with his mother.
Edited to say:
I looked on GrammarBook.com and that website mentions how both commas and periods are "the most frequently used punctuation marks" and that commas "customarily indicate a brief pause" without being as final as using a period. I think that speaks to what I mentioned about using commas for effect (and style too). I suggest being careful not to overuse commas though as this can bog a passage down (meaning, an indication that the passage could be "too wordy" and may start to become confusing)...see Rule #3a, #3b, and #3c.
Rules and examples provided by the same website:
1. Use commas to separate words and word groups in a simple series of three or more items
- Ex- My estate goes to my husband, son, daughter-in-law, and nephew.
- Ex- We had coffee, cheese and crackers, and grapes.
2. Use commas to separate two adjectives when the order of the adjective is interchangeable
Example: He is a strong, healthy man.
We could say healthy, strong man.
Example: We stayed at an expensive summer resort.
We would not say summer expensive resort, so no comma.
3a. Many inexperienced writers run two independent clauses together by using a comma instead of a period. This results in the dreaded run-on sentence or, more technically, a comma splice.
Incorrect: He walked all the way home, he shut the door.
He walked all the way home. He shut the door.
After he walked all the way home, he shut the door.
He walked all the way home, and he shut the door.
3b. In sentences where two independent clauses are joined by connectors such as and, or, but, etc., put a comma at the end of the first clause.
- Incorrect: He walked all the way home and he shut the door.
- Correct: He walked all the way home, and he shut the door.
Some writers omit the comma if the clauses are both quite short. (speaks to style, flow, also the effect aspect that I mentioned)
3c. If the subject does not appear in front of the second verb, a comma is generally unnecessary.
Example: He thought quickly but still did not answer correctly.
It went on to explain how a comma is needed to avoid confusion at times and used this example:
- Confusing: I saw that she was busy and prepared to leave.
- Clearer with comma: I saw that she was busy, and prepared to leave.