Welcome to Under Exposure and the World of Negative Film!
Both images are under exposed. First image, probably grossly so. See the strong backlight in the background? That threw your meter way off. Even more modern and sophisticated multi segment "matrix" meters would be off, but the simple center weighted "averaging" meter you have is thrown massively off by situations like this. It sees all that light in the background, and in the process of averaging the exposure over the entire image area, gives an exposure way too low for the important part of the image - your sons face.
Second image has a similar but less drastic situation. Your sons face is in reality a highlight, that is, it is brighter than middle grey. Same for the walls in the background. But your meter does not know this, and believes the boys face should be a brightness similar to middle grey. so instead of correct exposure, it only sets enought to make his face a dull grey level of brightness - and the shadow side of his face falls into murky, grainy darkness. Caucasiioan skin tones (tones = brightness, or at least it is helpful to think of it that way) should be 1 stop Above middle grey brightness. So, the fix is to add 1 stop of extra exposure in a sitaution like this. Plus, negative film is always improved by an extra stop, so plus 2 stops would be the best for your second image.
So, that is the Under Exposure part of the Story. And with Porta 400, even in 35mm, that is the greatest part of the story. Expose it well (make sure you err on the side of over exposure with negative film), and you will get much much better scans.
But the other part of the story is that negative film just simply scans more grainy and less sharp than slide film. Peroid. Plus, consumer scanners are optimized for slide film which makes the situation worse when home scanning negs (not your case, you had a minilab which was optimized for negtive scans do your work).
Try a roll of Provia 100F or Provia 400X and you will be amazed at how much better the scanned results are. Like worlds better. Sharper, with almost non-existant grain that allows digital like levels of inage sharpening if needed. Plus, you will be blown away by the colors and vibrancy of your slides! If you have an old family projector, your first slide show will be something you will not soon forget.
However, your metering must be much more precise. a non issue really with any of the multi segment metering cameras from the 1990's, and even point and shoots from that era. But your camera will require more care and thought in basing exposure readings.
But, I still highly recommend trying a roll of slide. It offers maximum differentiation to digital. Its a true analog expereince. Neg only approaches this level of uniqueness if you are doing optical printing, which is an art unto itself....