Assuming that the boy is born around 1118-20 (Henry V and Matilda marry in 1114 when Matilda is 12), he would probably be too young for serious consideration for the Empire if his father dies on schedule in 1125. The only non-adult chosen as Emperor by that date had been the infant Otto III, son of the deceased Otto II, at a time when the hereditary principle and centralization of the ruling dynasty and noble elite around their family was stronger than in 1125. Lothar of Supplinberg, as an adult and leading dynast within central Germany , would be chosen as Emperor over the under-age son of the late ruler in 1125, as in OTL, but Henry (VI) - if not ruling England as of 1135 - would be a probability for election after Lothar's death as hereditary leader of the Salian house; his direct male line would have priority over the Hohenstaufen who were descended from the Salians by the male line. So Frederick Barbarossa would not be Emperor in 1152, and his family would be the leading regional deputies of their Salian cousins .
If the young Henry is with his grandfather Henry I in Normandy when he dies on schedule in December 1135 and he is aged 15-17, he would be old enough to rule by contemporary reckoning and not yet in possession of more than a duchy in Germany. Many Anglo-Norman nobles might prefer him to the idea of a woman ruler and so 'leapfrog' Matilda. The latter is supposed to have been abrasive and confrontational and made enemies unnecessarily, as in London in 1141 in OTL, though this may be partly just contemporary attitudes to a strong woman ruler. Henry I forced his nobles to swear an oath to Matilda in 1126 in real life, and if Henry V had died and he had got Matilda to bring their son to England as his intended heir swearing an oath to a small boy in 1126 might have raised eyebrows too and increased support for a rival (probably Stephen, already adult and a leading magnate). In 1199 most nobles were to agree to pass over the nearest blood heir to Richard I, Arthur aged 12, for the adult John.
By 1135 Matilda's son would have been less controversial as he was older, unless the nobles (or Henry I) baulked at the idea of a partly absentee ruler who would spend much of his time in Germany . I can see Henry I pressurizing Matilda to remarry if Henry V had died on schedule leaving only one son, so he could get extra heirs - and the point of the Anjou marriage was partly a reconciliation between England/ Normandy and its current enemy Anjou. So Henry could still have married off Matilda to Geoffrey in 1128/9 and she would have had children by him (eldest son , b 1133, now called Geoffrey not Henry?) but her German son would have been Henry I's heir.
If Stephen uses nobles' fear of the German contender being lured off to Germany to take the throne there (and/or bringing in German nobles to claim land and jobs in England) to stage a coup in 1135 as in OTL,then Emperor Lothar dies on schedule in Dec 1137, what then? I can see that this would bring Henry V's and Matilda's son Henry to intervene in Germany and try to get elected Emperor, presumably in a struggle with Duke Henry of Saxony/ Bavaria the head of the Salians' Welf rivals. Whether he succeeds or ends up in a civil war there, he will be too preoccupied to join Matilda when she invades England to start up the full-scale civil war there in 1139; nor will the anti-Stephen nobles want him. So Matilda would be more likely to claim the crown for herself or for any son she had by a second marriage. If she had had two sons by Henry v that would be easier - one could have England, one Germany. That way we would have a Salian rather tha Hohenstaufen emperor in the 1150s-90s, and the probability of his backing up his brother or half-brother in England against the Kings of France when required. So whether the King of England had married Eleanor of Aquitaine and gained her dominions or else was just ruling Normandy (and Anjou?), the Anglo-German alliance could probably hold off the threat of Philip Augustus reconquering it in 1203-4 by a German attack on Paris. France would have stayed more disunited for longer. Nor would the German emperor be such a rival of England's Kings that he decided to kidnap a vulnerable travelling Crusader English King (Richard I in OTL) and force him to become his vassal in 1193-4; the two men would have been close relatives and allies.
