What Will My Son Look Like? See Your Future Boy

One chromosome settles the question of sex, and it comes from the father. A son inherits a Y from his father and a single X from his mother; the father's X goes only to his daughters. The SRY gene on that Y switches on male development within the first weeks of pregnancy.
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Everything about his face, though, is polygenic — face shape, nose, lips and jaw each draw on dozens of variants from both sides, so a son can favour his mother's features as readily as his father's. What changes is the timing. Testosterone through puberty lengthens the jaw and heavies the brow, which is why a boy's resemblance to his father often reads more strongly at sixteen than it did at six months.
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Height is where the two parents combine most predictably. The mid-parental method used in paediatric growth checks adds about 13 cm (5 inches) to the mother's height and averages that with the father's; most boys land within roughly 10 cm either side of the result. It gives you a range, not a number, and nutrition, health and the timing of puberty all move it.
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His single X has one further consequence. Anything recessive on that chromosome has no second copy to mask it, which is why red-green colour vision deficiency reaches roughly 1 in 12 men but only about 1 in 200 women — and why it travels down the mother's line.

Half true, and the half that is wrong matters. The strongest single locus for male-pattern baldness is AR, the androgen receptor gene, and it sits on the X chromosome — the one a son gets from his mother. That is where the advice to look at your mother's father comes from.
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But baldness is nowhere near a one-gene trait. The largest study so far, a UK Biobank analysis of 205,327 men, mapped 624 independent loci; the 26 on the X chromosome together accounted for about 12% of the heritability it could measure. The remaining ~88% sits on the other chromosomes, which arrive from both parents in equal measure.
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So a maternal grandfather with a full head of hair is weak reassurance, and the father's hairline is real information. The same one-gene-one-parent shortcut gets applied to eye colour, hair colour and height, and it is wrong in the same way each time: these are traits where many genes each contribute a little. If you want the mechanics, start with dominant and recessive traits.